Apply bugprone-stringview-nullptr.

Bug: None
Change-Id: Id6372a8a94f512fede50cdb36dc3ae378f271cee
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/258139
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36493}
diff --git a/pc/rtc_stats_collector.cc b/pc/rtc_stats_collector.cc
index c965161..6f10334 100644
--- a/pc/rtc_stats_collector.cc
+++ b/pc/rtc_stats_collector.cc
@@ -280,7 +280,7 @@
       break;
   }
   RTC_DCHECK_NOTREACHED();
-  return nullptr;
+  return {};
 }
 
 const char* QualityLimitationReasonToRTCQualityLimitationReason(