Add dummy return value to silence "function does not return a value in all

control paths" warning.
`ABSL_ASSUME(false)` should have been enough, but it doesn't seem to be on some
toolchains.

PiperOrigin-RevId: 532618766

src/google/protobuf/map_field.cc

@@ -60,6 +60,7 @@ VariantKey RealKeyToVariantKey<MapKey>::operator()(const MapKey& value) const {
       return VariantKey(static_cast<uint64_t>(value.GetBoolValue()));
     default:
       ABSL_ASSUME(false);
+      return VariantKey(uint64_t{});
   }
 }