Fix DCHECK when encoding GenericPacket* events using the legacy RTC event log format.

Bug: webrtc:10319
Change-Id: I6764072281b8d66a8e4a5b2c0e547af3bfddda37
Reviewed-on: https://webrtc-review.googlesource.com/c/123121
Commit-Queue: Peter Slatala <psla@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26693}

logging/rtc_event_log/encoder/rtc_event_log_encoder_legacy.cc

@@ -365,8 +365,8 @@ std::string RtcEventLogEncoderLegacy::Encode(const RtcEvent& event) {
     case RtcEvent::Type::GenericPacketReceived:
     case RtcEvent::Type::GenericPacketSent:
     case RtcEvent::Type::GenericAckReceived:
-      // These are unsupported in the old format.
-      break;
+      // These are unsupported in the old format, but shouldn't crash.
+      return "";
   }
 
   int event_type = static_cast<int>(event.GetType());