Since PHP 8.1, passing a null string to `trim()` is deprecated.
Thus first check that `$content` is not null before trimming it by using `phutil_nonempty_string()` as a replacement for `strlen()`.
Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.
```
EXCEPTION: (RuntimeException) trim(): Passing null to parameter #1 ($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