Double check for events after parsing report data

In the case that event files are found but cannot be read or
parsed, we might write a session with no events. To mitigate this,
we will double-check to make sure we've parsed the events properly
before synthesizing a report file.

Author: mrwillis21

firebase-crashlytics/src/main/java/com/google/firebase/crashlytics/internal/persistence/CrashlyticsReportPersistence.java

@@ -306,6 +306,12 @@ private void synthesizeReport(@NonNull File sessionDirectory, long sessionEndTim
       }
     }
 
+    // b/168902195
+    if (events.isEmpty()) {
+      Logger.getLogger().d("Could not parse event files for session " + sessionDirectory.getName());
+      return;
+    }
+
     String userId = null;
     final File userIdFile = new File(sessionDirectory, USER_FILE_NAME);
     if (userIdFile.isFile()) {