Fix returning nullptr where now a std::string_view is required

mp4/mp4ids.cpp

@@ -450,7 +450,7 @@ std::string_view channelConfigString(std::uint8_t config)
     case FrontCenterFrontLeftFrontRightSideLeftSideRightBackLeftBackRightLFEChannel:
         return "8 channels: front-center, front-left, front-right, side-left, side-right, back-left, back-right, LFE-channel";
     default:
-        return nullptr;
+        return std::string_view();
     }
 }
 

mpegaudio/mpegaudioframe.cpp

@@ -26,7 +26,7 @@ std::string_view mpegChannelModeString(MpegChannelMode channelMode)
     case MpegChannelMode::SingleChannel:
         return "1 channel: single channel";
     default:
-        return nullptr;
+        return std::string_view();
     }
 }