Hide Initial Clip property if not relevant in AudioStreamInteractive editor

[Calinou]

• Follow-up to Expand columns in the audio interactive transition editor #95134 (can be merged independently).

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

PS: Is there a way to force the parameter list to be updated? This would allow Switch to Clip to be hidden if there are no clips as well:

godot/modules/interactive_music/audio_stream_interactive.cpp

Line 458 in 3978628

r_parameters->push_back(Parameter(PropertyInfo(Variant::STRING, "switch_to_clip", PROPERTY_HINT_ENUM, clip_names, PROPERTY_USAGE_EDITOR), ""));

The following code works, but it doesn't update when you add/remove a clip until you reselect the node:

r_parameters->push_back(Parameter(PropertyInfo(Variant::STRING, "switch_to_clip", PROPERTY_HINT_ENUM, clip_names, get_clip_count() == 0 ? PROPERTY_USAGE_NO_EDITOR : PROPERTY_USAGE_EDITOR), ""));

Fixing this would allow fully closing #94140 when coupled with #95134.

• This partially addresses AudioStreamInteractive does not scale with editor & other minor visual issues #94140.

Preview

audio_interactive_editor_initial_clip.mp4