Fix PHP 8.1 "strlen(null)" exception about Staging URI on Diffusion repo History page

Summary:
strlen() was used in Phabricator to check if a generic value is a non-empty string.
Passing null to strlen() is deprecated since PHP 8.1. Thus first check that the string is not null.

EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=b325304b6e52), phorge(head=master, ref.master=83edbffd521d)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/repository/xaction/PhabricatorRepositoryStagingURITransaction.php:20]

This is quite similar to D25277.

Closes T15458

Test Plan: After applying this change, editing the Staging URI by saving its already empty value, and going to the repo History page, the page /diffusion/1/manage/history/ renders without an exception, showing R1 being Inactive and user set as the staging area for this repository. [sic!]

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15458

Differential Revision: https://we.phorge.it/D25282