R1:5405134fa5db

Fix PHP 8.1 "strlen(null)" exception which can block repository imports

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1.

Interestingly, in upstream they started fixing this yesterday, with just this minimal change:

https://secure.phabricator.com/rPf6214f060e780ecf7b565c5a0edbd28d85c03275#C11580NL1139

Premising counting the length of a string just to answer the question "is this empty?" may be overkill,
but…