Fix PHP 8.1 "strlen(null)" exceptions creating a Diffusion Identity without entering assignee
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.
```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=b325304b6e52), phorge(head=diffusionCreateIdentity, ref.master=cbc0e661544a, ref.diffusionCreateIdentity=cbc0e661544a)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/repository/xaction/PhabricatorRepositoryIdentityAssignTransaction.php:55]
```
```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=b325304b6e52), phorge(head=diffusionCreateIdentity, ref.master=cbc0e661544a, ref.diffusionCreateIdentity=cbc0e661544a)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/infrastructure/storage/lisk/PhabricatorLiskDAO.php:298]
```
```
EXCEPTION: (RuntimeException) mb_detect_encoding(): 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=diffusionCreateIdentity, ref.master=cbc0e661544a, ref.diffusionCreateIdentity=cbc0e661544a)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
#1 <#2> mb_detect_encoding(NULL, array) called at [<phorge>/src/infrastructure/storage/lisk/PhabricatorLiskDAO.php:313]
```
Closes T15440
Test Plan: After applying these three changes, going to `/diffusion/identity/edit/form/default/` and selecting the "Create Identity" button will only show a AphrontQueryException unrelated to PHP 8 and covered in T15439 instead.
Reviewers: O1 Blessed Committers, speck
Reviewed By: O1 Blessed Committers, speck
Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15440
Differential Revision: https://we.phorge.it/D25271