[NFC] Fix null pointer profiler warning (#125)

CHANGELOG.md

@@ -1,3 +1,6 @@
+# Unreleased
+- Fix null pointer warning from profiler. (#124)
+
 # v9.4.1 (CocoaPods Only)
 - The v9.4.0 podspec did not include the intended dependency range for nanopb
   ([firebase-ios-sdk/#12477](https://github.com/firebase/firebase-ios-sdk/issues/12477))

GoogleDataTransport/GDTCORLibrary/GDTCORLogSourceMetrics.m

@@ -52,7 +52,7 @@ typedef NSDictionary<NSNumber *, NSNumber *> GDTCORDroppedEventCounter;
     // If the dropped event counter for this event's mapping ID is `nil`,
     // an empty mutable counter is returned.
     NSMutableDictionary<NSNumber *, NSNumber *> *eventCounter =
-        [NSMutableDictionary dictionaryWithDictionary:eventCounterByLogSource[event.mappingID]];
+        [NSMutableDictionary dictionaryWithDictionary:eventCounterByLogSource[event.mappingID] ?: @{}];
 
     // Increment the log source metrics for the given reason.
     NSInteger currentEventCountForReason = [eventCounter[@(reason)] integerValue];