Fix Navigator crashing on ledgers with no packages (#3400)

navigator/backend/src/main/scala/com/digitalasset/navigator/model/PackageRegistry.scala

@@ -52,12 +52,10 @@ case class PackageRegistry(
       }
 
     val newTemplates = newPackages
-      .map(_._2.templates)
-      .reduce(_ ++ _)
+      .flatMap(_._2.templates)
 
     val newTypeDefs = newPackages
-      .map(_._2.typeDefs)
-      .reduce(_ ++ _)
+      .flatMap(_._2.typeDefs)
 
     copy(
       packages = packages ++ newPackages,

unreleased.rst

@@ -15,3 +15,4 @@ HEAD — ongoing
   that allows you to mark contracts as pending. Those contracts will
   be automatically filtered from the result of ``getContracts`` until
   we receive the corresponding completion/transaction.
+- [Navigator] Fixed a bug where Navigator becomes unresponsive if the ledger does not contain any DAML packages.