Hide Initial Clip property if not relevant in AudioStreamInteractive editor

If no clips have been created yet, there's no clip to choose from
for the Initial Clip property.

modules/interactive_music/audio_stream_interactive.cpp

@@ -431,6 +431,10 @@ void AudioStreamInteractive::_validate_property(PropertyInfo &r_property) const
 
 	if (prop == "initial_clip") {
 #ifdef TOOLS_ENABLED
+		if (get_clip_count() == 0) {
+			// Hide property since there's no clip to choose from.
+			r_property.usage = PROPERTY_USAGE_NO_EDITOR;
+		}
 		r_property.hint_string = _get_streams_hint();
 #endif
 	} else if (prop.begins_with("clip_") && prop != "clip_count") {