fix: avoid uiTextDisplayer.destroy crashing if called more than once

[antondc]

Currently an instance of uiTextDisplayer is not idempotent, crashing if uiTextDisplayer.destroy() is called more than once:

const uiTextDisplayer = new shaka.text.UITextDisplayer(
  videoElement,
  subtitlesElement
);

uiTextDisplayer.destroy();
uiTextDisplayer.destroy();

This is due to videoContainer.removeChild being called with null parameter on the second interation, as textContainer was already deleted on the first one.
Source:

shaka-player/lib/text/ui_text_displayer.js

Line 158 in 9f5e461

this.videoContainer_.removeChild(this.textContainer_);

An early return will prevent this crash, maintaining consistency with other methods.

Already created an issue for this bug at: #6023