Fix PHP 8.1 "ltrim(null)" exception which blocks rendering first Meme comment

Summary:
Since PHP 8.1, passing a null string to ltrim(string $string) is deprecated.

Thus first check if After and Below text are not null before trimming.

Closes T15379

Test Plan:
Applied this change; afterwards several times created a new Pholio mock and added a Meme comment. I could not reproduce the problem anymore (first meme comment in a mock always rendered correctly instead of showing an exception as comment). However, I could not reliably reproduce the problem anyway.

• Create a meme without any above/below text
• Create a meme with just spaces as above/below text
• Create a meme with just above text as "asd", or "0" or a lizard
• Create a meme with just below text as "asd", or "0" or a lizard
• Create a meme with both above and below texts with "asd", "0" and a lizard and more stuff (doing all 64 combinations)

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15379

Differential Revision: https://we.phorge.it/D25212