Switch to using new ACM methods for encoder management

webrtc/modules/audio_coding/acm2/rent_a_codec.cc

Index: webrtc/modules/audio_coding/acm2/rent_a_codec.cc
diff --git a/webrtc/modules/audio_coding/acm2/rent_a_codec.cc b/webrtc/modules/audio_coding/acm2/rent_a_codec.cc
index 3bf959e901eab5a297be6256e22e0d2914668242..7f1e52030dc944f9efe244d8fd7f96eec432ebc8 100644
--- a/webrtc/modules/audio_coding/acm2/rent_a_codec.cc
+++ b/webrtc/modules/audio_coding/acm2/rent_a_codec.cc
@@ -263,7 +263,8 @@ RentACodec::StackParameters::~StackParameters() = default;
 
 std::unique_ptr<AudioEncoder> RentACodec::RentEncoderStack(
     StackParameters* param) {
-  RTC_DCHECK(param->speech_encoder);
+  if (!param->speech_encoder)
+    return nullptr;
 
   if (param->use_codec_fec) {
     // Switch FEC on. On failure, remember that FEC is off.