Check for user ID file before trying to read it (firebase#1635)

The user ID file will be missing if no user ID was set, so check if it exists before trying to read from it to avoid unnecessary error logging. Fixes one of the prevalent issues in firebase#1559

Author: mrwillis21

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

@@ -305,10 +305,13 @@ private void synthesizeReport(@NonNull File sessionDirectory, long sessionEndTim
  }
 
  String userId = null;
- try {
- userId = readTextFile(new File(sessionDirectory, USER_FILE_NAME));
- } catch (IOException e) {
- Logger.getLogger().d("Could not read user ID file in " + sessionDirectory.getName(), e);
+ final File userIdFile = new File(sessionDirectory, USER_FILE_NAME);
+ if (userIdFile.isFile()) {
+ try {
+ userId = readTextFile(userIdFile);
+ } catch (IOException e) {
+ Logger.getLogger().d("Could not read user ID file in " + sessionDirectory.getName(), e);
+ }
  }
 
  final File reportFile = new File(sessionDirectory, REPORT_FILE_NAME);