Fixing some of the issues found by clang static analyzer.

Bug: webrtc:8737
Change-Id: Ib436449c493336e7c35a72a96dc88cccdbb5bbaf
Reviewed-on: https://webrtc-review.googlesource.com/39200
Commit-Queue: Peter Hanspers <peterhanspers@webrtc.org>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21607}
diff --git a/sdk/objc/Framework/Classes/Common/RTCFileLogger.mm b/sdk/objc/Framework/Classes/Common/RTCFileLogger.mm
index 15461f4..6acb942 100644
--- a/sdk/objc/Framework/Classes/Common/RTCFileLogger.mm
+++ b/sdk/objc/Framework/Classes/Common/RTCFileLogger.mm
@@ -124,7 +124,7 @@
   _logSink.reset();
 }
 
-- (NSData *)logData {
+- (nullable NSData *)logData {
   if (_hasStarted) {
     return nil;
   }