Empty text background texture appears when clearing actionbar

Partner Rank: 3

Verification builds: 1.16.0.63
Regression builds: 1.16.0.61

Summary: Currently, /title actionbar messages always have a background, regardless of the new accessibility setting (see MCPE-79331). The issue is that this background appears even when sending a blank actionbar message.

Impact: An upcoming map uses the actionbar text for subtitles. When a line is finished, we clear the text so it doesn't persist. Because of this issue, the message leaves behind a gray box for ten seconds, which is distracting but not gamebreaking by any means.

Repro Steps:

1. /title @a actionbar

Expected Results: Nothing should happen if the actionbar text is already blank, otherwise it should clear it.

Observed Results: Gray rectangle appears on the screen at the actionbar slot.

Screenshots/Videos attached: Yes.

Notes: The impacted map changes the default duration of actionbar messages from 3 seconds to 10 seconds, because some subtitled lines are quite long. This exacerbates the impact of the visible box. There's also a section where the actionbar is constantly cleared, resulting in a constant gray box now.