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}

sdk/objc/components/video_codec/RTCVideoEncoderH264.mm

@@ -753,7 +753,7 @@ CFStringRef ExtractProfile(webrtc::SdpVideoFormat videoFormat) {
   _bitrateAdjuster->Update(frame.buffer.length);
 }
 
-- (RTCVideoEncoderQpThresholds *)scalingSettings {
+- (nullable RTCVideoEncoderQpThresholds *)scalingSettings {
   return [[RTCVideoEncoderQpThresholds alloc] initWithThresholdsLow:kLowH264QpThreshold
                                                                high:kHighH264QpThreshold];
 }