Allow messages to be null in recording logger

So that we know what's going on with spurious failures like:

```
[error] Test bloop.tasks.TestLoggingSpec.concurrentTestRunsHaveDifferentStreams failed: java.lang.NullPointerException: null, took 4.542 sec
[error]     at scala.collection.immutable.StringLike.stripSuffix(StringLike.scala:165)
[error]     at scala.collection.immutable.StringLike.stripSuffix$(StringLike.scala:164)
[error]     at scala.collection.immutable.StringOps.stripSuffix(StringOps.scala:29)
[error]     at bloop.logging.RecordingLogger.$anonfun$getMessages$1(RecordingLogger.scala:12)
[error]     at scala.collection.immutable.List.map(List.scala:287)
[error]     at bloop.logging.RecordingLogger.getMessages(RecordingLogger.scala:10)
[error]     at bloop.tasks.TestLoggingSpec.concurrentTestRunsHaveDifferentStreams(TestLoggingSpec.scala:52)
[error] ...
```

frontend/src/test/scala/bloop/logging/RecordingLogger.scala

@@ -9,7 +9,9 @@ class RecordingLogger extends AbstractLogger {
   def clear(): Unit = messages.clear()
   def getMessages(): List[(String, String)] = messages.iterator.asScala.toList.map {
     // Remove trailing '\r' so that we don't have to special case for Windows
-    case (category, msg) => (category, msg.stripSuffix("\r"))
+    case (category, msg0) =>
+      val msg = if (msg0 == null) "<null reference>" else msg0
+      (category, msg.stripSuffix("\r"))
   }
 
   override val name: String = "RecordingLogger"