Fix PHP 8.1 "strlen(null)" exceptions in Phame transactions
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. Phorge adopts `phutil_nonempty_string()` as a replacement.
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.
```
strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [PhameBlogParentDomainTransaction.php:18]
strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [PhameBlogFullDomainTransaction.php:25]
```
Ref T15064
Test Plan: Unknown, presumably change the parent domain of a Phame blog. These only became visible after D26732 was applied.
Reviewers: O1 Blessed Committers, aklapper
Reviewed By: O1 Blessed Committers, aklapper
Subscribers: aklapper, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15064
Differential Revision: https://we.phorge.it/D26737