postgresql-testing: On failure, logs are no longer an Option. (#5190)

`logs` used to be an `Option[Seq[String]]`. They were changed to
`Seq[String]` with a default of `Seq.empty`, but the code to print them
still assumed an `Option`, which means they were printing as
"ArrayBuffer(line 1, line 2, line 3)" instead of, well, nicely.

CHANGELOG_BEGIN
CHANGELOG_END

libs-scala/postgresql-testing/src/main/scala/com/digitalasset/testing/postgresql/PostgresAround.scala

@@ -209,7 +209,7 @@ object PostgresAround {
           command.mkString("\n"),
           s"\nSTDOUT:\n$stdout",
           s"\nSTDERR:\n$stderr",
-          logs.map(lines => s"\nLogs:\n${lines.mkString("\n")}"),
+          if (logs.isEmpty) "\nLogs: <none>" else s"\nLogs:\n${logs.mkString("\n")}",
         ).mkString("\n"),
         cause,
       )