R1:3999a286741d

Fix PHP 8.1 exceptions which block adding an embedded File preview as a Comment

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

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific…