PHP 8.2: fixes for strlen() not accepting NULL anymore, part 1

Summary:
This change avoids some unnecessary uses of the strlen() function,
actually fixing some deprecation warnings in PHP 8.2.

In short, this is the suggested universal replace:

-if(strlen($v))
+if(phutil_nonempty_string($v))

And, if you know PHP, this is also another adoptable replace, but
only for cases where you are sure that the string "0" is not useful:

-if(strlen($v))
+if($v))

As usual the optimal solution depends on the contest.

Other similar patches will probably follow.

Closes T15222
Ref T15190

Test Plan:

• for the first time in my life, with this change, the unit tests are passed in PHP 8.2
• check with your big eyes that there are no obvious typos

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15199, T15190, T15222

Differential Revision: https://we.phorge.it/D25104