fix abortMessage check

Author: supervacuus

sentry-android-core/src/main/java/io/sentry/android/core/internal/tombstone/TombstoneParser.java

@@ -172,7 +172,7 @@ private Message constructMessage(@NonNull final TombstoneProtos.Tombstone tombst
           String.format(
               Locale.ROOT,
               "%sFatal signal %s (%d), %s (%d), pid = %d (%s)",
-              abortMessage != null ? abortMessage + ": " : "",
+              !abortMessage.isEmpty() ? abortMessage + ": " : "",
               signalInfo.getName(),
               signalInfo.getNumber(),
               signalInfo.getCodeName(),