Add a check for empty channels to ChannelManager dtor.

This is a necessary precondition for upcoming changes that will remove
calls to Invoke() that currently occur during construction/destruction
of media channel objects.

Subsequently fix RtpSenderReceiverTest to destroy channels that were
created in the constructor, in the destructor.

Bug: webrtc:11992
Change-Id: I526908d997d27495659805d84113c84c48568712
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/246680
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35702}

pc/rtp_sender_receiver_unittest.cc

@@ -169,6 +169,10 @@ class RtpSenderReceiverTest
     local_stream_ = nullptr;
     video_track_ = nullptr;
     audio_track_ = nullptr;
+
+    channel_manager_->DestroyVoiceChannel(voice_channel_);
+    channel_manager_->DestroyVideoChannel(video_channel_);
+
     worker_thread_->Invoke<void>(RTC_FROM_HERE,
                                  [&]() { channel_manager_.reset(); });
   }