The Bug

An empty sound means, in the sounds json, the "sounds" of it is undefined. It will be regarded valid, because it can be in the tab-complete list in the /playsound cmd. But, it displays no subtitle.
Also, /playsound a valid sound event while setting the volumn parameter 0, the subtitle will not be shown. That means, it seems impossible to show a subtitle without playing a sound.

Reproduce

/playsound minecraft:block.note_block.banjo block @s ~ ~ ~ 0 1

I can make sure I opened displaying subtitles in the option. If the volume is 0.1, the subtitle will show.

Expected Result

A subtitle would show