Fix PHP 8.1 "trim(null)" exception which blocks rendering Conduit's harbormaster.sendmessage page

Summary:
Since PHP 8.1, passing a null string to trim() is deprecated.
Thus first check that $content is not null before trimming it.

Also since trim() returns a string and never null, we can also simplify the non-empty check,
in a more readable and efficient way, avoiding strlen() that was usually used for other "more wild" cases.

EXCEPTION: (RuntimeException) trim(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=18554ea76ceb), phorge(head=master, ref.master=0d81da590923)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
  #1 <#2> trim(NULL) called at [<phorge>/src/infrastructure/markup/blockrule/PhutilRemarkupTableBlockRule.php:117]

Closes T15427

Test Plan: Applied this change, afterwards /conduit/method/harbormaster.sendmessage/ correctly rendered in web browser.

Reviewers: O1 Blessed Committers, speck, valerio.bozzolan

Reviewed By: O1 Blessed Committers, speck, valerio.bozzolan

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

Maniphest Tasks: T15427

Differential Revision: https://we.phorge.it/D25259