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