Fix "Undefined index" exception setting Meme text

Summary:
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 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 corner case.

EXCEPTION: (RuntimeException) Undefined index: above at [<arcanist>/src/error/PhutilErrorHandler.php:251]
 arcanist(), phorge()
   #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phorge>/src/applications/macro/engine/PhabricatorMemeEngine.php:276]

Closes T15637

Test Plan:
Create a meme called "angrycat" from the /macro/ page, and try a comment like this, expecting no nuclear implosion:

{meme, src=angrycat, below=}
{meme, src=angrycat, above=}
{meme, src=angrycat, below=, above=}
{meme, src=angrycat, below=  , above=  }
{meme, src=angrycat, below=asd}
{meme, src=angrycat, above=asd}
{meme, src=angrycat, above=asd, below=dsa}
{meme, src=angrycat, above=   asd   , below=   dsa  }

Also carefully read the code with your big eyes, keeping in mind that strlen does not accept passing null in PHP 8, and looking at what we did in rPb4cfe56f03b44615ac9251aed8d74bf13b085051.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15637

Differential Revision: https://we.phorge.it/D25437