R1:76ef9db8a5ed

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…