R1:f45d6421a13c

Fix PHP 8.1 "strlen(null)" exception opening a File overlay (Lightbox comment frame)

Summary:
The error was caused by the overlay lightbox, since it gives the possibility to share a comment on any file
in any general Remarkup text, but that comment textarea is indeed empty as default.

`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts `phutil_nonempty_string()` as a replacement.

Note: this may highlight other…