Block Editor: Fix PHP warning when loading editor styles while in RTL.

In RTL languages, WordPress adds `style-editor-rtl.css` editor styles to the global `$editor_styles`.
This patch ignores handling these styles if the file is not preset.

Also, clarify the docs for the return value of the `block_version` function.

Props mostafa.s1990, desrosj, mukesh27.

Merges [43923] and [43924] to trunk.

Fixes #45288, #45342.
Built from https://develop.svn.wordpress.org/trunk@44270


git-svn-id: https://core.svn.wordpress.org/trunk@44100 1a063a9b-81f0-0310-95a4-ce76da25c4cd