`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.
Also fix similar warning for `ctype_digit(): Argument of type null will be interpreted as string in the future` by checking for `null` first.
```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=customOAuthUrlencodeNull, ref.master=788098096e11, ref.customOAuthUrlencodeNull=4f0f2043b7e9), phorge(head=master, ref.master=bcfcd9acfc12)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php:27]
```
```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=customOAuthUrlencodeNull, ref.master=788098096e11, ref.customOAuthUrlencodeNull=4f0f2043b7e9), phorge(head=customFieldDate, ref.master=bcfcd9acfc12, ref.customFieldDate=bcfcd9acfc12)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php:35]
```
```
EXCEPTION: (RuntimeException) ctype_digit(): Argument of type null will be interpreted as string in the future at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=customOAuthUrlencodeNull, ref.master=788098096e11, ref.customOAuthUrlencodeNull=4f0f2043b7e9), phorge(head=customFieldDate, ref.master=bcfcd9acfc12, ref.customFieldDate=bcfcd9acfc12)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
#1 <#2> ctype_digit(NULL) called at [<phorge>/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php:77]
```
Closes T15601