iOS/MacOS:Refactor metal rendering by extracting common implementation

webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLNSVideoView.m

Index: webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLNSVideoView.m
diff --git a/webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLNSVideoView.m b/webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLNSVideoView.m
index 7c7084149311746868e7bec9f7b5c4bdc6716fa0..dd413932676c80434edfee918d5e1c100878779f 100644
--- a/webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLNSVideoView.m
+++ b/webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLNSVideoView.m
@@ -17,7 +17,7 @@
 
 #import "RTCMTLI420Renderer.h"
 
-@interface RTCMTLNSVideoView () <MTKViewDelegate>
+@interface RTCMTLNSVideoView ()<MTKViewDelegate>
 @property(nonatomic) id<RTCMTLRenderer> renderer;
 @property(nonatomic, strong) MTKView *metalView;
 @property(atomic, strong) RTCVideoFrame *videoFrame;
@@ -90,6 +90,9 @@
 
 #pragma mark - MTKViewDelegate methods
 - (void)drawInMTKView:(nonnull MTKView *)view {
+  if (self.videoFrame == nil) {
+    return;
+  }
   if (view == self.metalView) {
     [_renderer drawFrame:self.videoFrame];
   }