Scaling settings nullability in Obj-C SDK.

This method is allowed to return nil but was not annotated as such.

Bug: webrtc:8560
Change-Id: If54aa94d6ff83b7bdb87b526244616e2627a8999
Reviewed-on: https://webrtc-review.googlesource.com/97380
Reviewed-by: Peter Hanspers <peterhanspers@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24568}
diff --git a/sdk/objc/base/RTCVideoEncoder.h b/sdk/objc/base/RTCVideoEncoder.h
index 41e78ba..b566615 100644
--- a/sdk/objc/base/RTCVideoEncoder.h
+++ b/sdk/objc/base/RTCVideoEncoder.h
@@ -42,7 +42,7 @@
 /** Returns QP scaling settings for encoder. The quality scaler adjusts the resolution in order to
  *  keep the QP from the encoded images within the given range. Returning nil from this function
  *  disables quality scaling. */
-- (RTCVideoEncoderQpThresholds *)scalingSettings;
+- (nullable RTCVideoEncoderQpThresholds *)scalingSettings;
 
 @end