Fix serialization of NPE in logger (#4055)

Fixes an error when the logger processes NPE:
```
[internal-logger-error] One of the printers failed to write a message: java.lang.NullPointerException
```

lib/scala/logging-service/src/main/scala/org/enso/loggingservice/internal/protocol/SerializedException.scala

@@ -16,7 +16,7 @@ import io.circe._
   */
 case class SerializedException(
   name: String,
-  message: String,
+  message: Option[String],
   stackTrace: Seq[SerializedException.TraceElement],
   cause: Option[SerializedException]
 )
@@ -33,7 +33,7 @@ object SerializedException {
   ): SerializedException =
     SerializedException(
       name       = name,
-      message    = message,
+      message    = Some(message),
       stackTrace = stackTrace,
       cause      = Some(cause)
     )
@@ -45,18 +45,13 @@ object SerializedException {
     message: String,
     stackTrace: Seq[SerializedException.TraceElement]
   ): SerializedException =
-    SerializedException(
+    new SerializedException(
       name       = name,
-      message    = message,
+      message    = Some(message),
       stackTrace = stackTrace,
       cause      = None
     )
 
-  /** Creates a [[SerializedException]] from a [[Throwable]].
-    */
-  def apply(throwable: Throwable): SerializedException =
-    fromException(throwable)
-
   /** Encodes a JVM [[Throwable]] as [[SerializedException]].
     */
   def fromException(throwable: Throwable): SerializedException = {
@@ -66,7 +61,7 @@ object SerializedException {
       else Option(throwable.getCause).map(fromException)
     SerializedException(
       name       = Option(clazz.getCanonicalName).getOrElse(clazz.getName),
-      message    = throwable.getMessage,
+      message    = Option(throwable.getMessage),
       stackTrace = throwable.getStackTrace.toSeq.map(encodeStackTraceElement),
       cause      = cause
     )
@@ -164,7 +159,7 @@ object SerializedException {
   ): Decoder.Result[SerializedException] = {
     for {
       name       <- json.get[String](JsonFields.Name)
-      message    <- json.get[String](JsonFields.Message)
+      message    <- json.get[Option[String]](JsonFields.Message)
       stackTrace <- json.get[Seq[TraceElement]](JsonFields.StackTrace)
       cause <-
         json.getOrElse[Option[SerializedException]](JsonFields.Cause)(None)

lib/scala/logging-service/src/test/scala/org/enso/loggingservice/internal/DefaultLogMessageRendererSpec.scala

@@ -0,0 +1,33 @@
+package org.enso.loggingservice.internal
+
+import org.enso.loggingservice.LogLevel
+import org.enso.loggingservice.internal.protocol.{
+  SerializedException,
+  WSLogMessage
+}
+import org.scalatest.OptionValues
+import org.scalatest.matchers.should.Matchers
+import org.scalatest.wordspec.AnyWordSpec
+
+import java.time.Instant
+import java.time.temporal.ChronoUnit
+
+class DefaultLogMessageRendererSpec
+    extends AnyWordSpec
+    with Matchers
+    with OptionValues {
+
+  "DefaultLogMessageRenderer" should {
+    "render NullPointerException" in {
+      val renderer = new DefaultLogMessageRenderer(printExceptions = true)
+      val ts       = Instant.now().truncatedTo(ChronoUnit.MILLIS)
+
+      val exception =
+        SerializedException.fromException(new NullPointerException)
+      val message =
+        WSLogMessage(LogLevel.Trace, ts, "group", "message", Some(exception))
+
+      noException should be thrownBy renderer.render(message)
+    }
+  }
+}

lib/scala/logging-service/src/test/scala/org/enso/loggingservice/internal/SerializedExceptionSpec.scala

@@ -10,6 +10,7 @@ class SerializedExceptionSpec
     extends AnyWordSpec
     with Matchers
     with OptionValues {
+
   "SerializedException" should {
     "serialize and deserialize with nested causes" in {
       val cause = SerializedException(
@@ -25,7 +26,7 @@ class SerializedExceptionSpec
           SerializedException.TraceElement("e2", "loc2"),
           SerializedException.TraceElement("e3", "loc3")
         ),
-        cause = cause
+        cause
       )
 
       exception.asJson
@@ -33,5 +34,15 @@ class SerializedExceptionSpec
         .toOption
         .value shouldEqual exception
     }
+
+    "be created from NullPointerException" in {
+      val exception = new NullPointerException()
+      val result    = SerializedException.fromException(exception)
+
+      result.name shouldEqual exception.getClass.getName
+      result.message shouldEqual None
+      result.cause shouldEqual None
+      result.stackTrace shouldBe Symbol("nonEmpty")
+    }
   }
 }

lib/scala/logging-service/src/test/scala/org/enso/loggingservice/internal/WSLogMessageSpec.scala

@@ -14,17 +14,31 @@ import org.scalatest.matchers.should.Matchers
 import org.scalatest.wordspec.AnyWordSpec
 
 class WSLogMessageSpec extends AnyWordSpec with Matchers with OptionValues {
+
   "WSLogMessage" should {
     "serialize and deserialize to the same thing" in {
       val ts = Instant.now().truncatedTo(ChronoUnit.MILLIS)
 
       val message1 = WSLogMessage(LogLevel.Trace, ts, "group", "message", None)
       message1.asJson.as[WSLogMessage].toOption.value shouldEqual message1
+      noException should be thrownBy message1.asJson.noSpaces
 
-      val exception = SerializedException("name", "message", Seq(), None)
+      val exception = SerializedException("name", "message", Seq())
       val message2 =
         WSLogMessage(LogLevel.Trace, ts, "group", "message", Some(exception))
       message2.asJson.as[WSLogMessage].toOption.value shouldEqual message2
+      noException should be thrownBy message2.asJson.noSpaces
+    }
+
+    "serialize NullPointerException" in {
+      val ts = Instant.now().truncatedTo(ChronoUnit.MILLIS)
+
+      val exception =
+        SerializedException.fromException(new NullPointerException)
+      val message =
+        WSLogMessage(LogLevel.Trace, ts, "group", "message", Some(exception))
+      message.asJson.as[WSLogMessage].toOption.value shouldEqual message
+      noException should be thrownBy message.asJson.noSpaces
     }
   }
 }