Empty MIDI clips are not highlighted on selection

On the image below, the top track is not selected, while the bottom track is. Note how the non-empty MIDI clips behave correctly, but the empty ones always look the same, selected or not. This makes things very confusing when selecting empty MIDI clips.

Reproduce the issue in LMMS by selecting tracks containing both empty and non-empty MIDI clips, using the attached image as the expected comparison. Trace the clip-selection rendering entry point and verify that empty clips visibly differ between selected and unselected states.

Written by the indexing model from the issue text.