Polishing code to handle certificate generation failure in .mm files.

This is a follow-up to https://codereview.webrtc.org/1965313002/ which
was TBR-landed.

Minor code clean-up/corrections:

Property nativeConfiguration -> - method createNativeConfiguration.
RTCLogWarning -> RTCLogError.
setConfiguration returning NO instead of false.
initWithFactory returning nil instead of nullptr.
Braces around ifs.

Review-Url: https://codereview.webrtc.org/1978233002
Cr-Commit-Position: refs/heads/master@{#12770}

webrtc/sdk/objc/Framework/UnitTests/RTCConfigurationTest.mm

@@ -45,7 +45,8 @@
       RTCContinualGatheringPolicyGatherContinually;
 
   std::unique_ptr<webrtc::PeerConnectionInterface::RTCConfiguration>
-      nativeConfig(config.nativeConfiguration);
+      nativeConfig([config createNativeConfiguration]);
+  EXPECT_TRUE(nativeConfig.get());
   EXPECT_EQ(1u, nativeConfig->servers.size());
   webrtc::PeerConnectionInterface::IceServer nativeServer =
       nativeConfig->servers.front();