Fix PHP 8.1 "strlen(null)" exceptions editing a form when custom field of type Date exists

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_scalar()` as a replacement when both string and integers could be passed as a value like here.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_scalar() 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 freebsd#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 <freebsd#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php:27]
```

```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter freebsd#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 <freebsd#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 <freebsd#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
  freebsd#1 <freebsd#2> ctype_digit(NULL) called at [<phorge>/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php:77]
```

Closes T15601

Test Plan: After applying these three changes and creating a custom field with `"type": "date"` under `/config/edit/maniphest.custom-field-definitions/`, the website `/transactions/editengine/maniphest.task/view/5/` renders correctly in the browser, showing "This is a preview of the current form configuration."

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15601

Differential Revision: https://we.phorge.it/D25389

Author: aklapper

Diff for: src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php

@@ -24,15 +24,15 @@ public function buildOrderIndex() {
 
   public function getValueForStorage() {
     $value = $this->getFieldValue();
-    if (strlen($value)) {
+    if (phutil_nonempty_scalar($value)) {
       return (int)$value;
     } else {
       return null;
     }
   }
 
   public function setValueFromStorage($value) {
-    if (strlen($value)) {
+    if (phutil_nonempty_scalar($value)) {
       $value = (int)$value;
     } else {
       $value = null;
@@ -74,7 +74,8 @@ private function newDateControl() {
     // specify as a string. Parse the string into an epoch.
 
     $value = $this->getFieldValue();
-    if (!ctype_digit($value)) {
+    if ($value !== null && gettype($value) !== 'integer' &&
+        !ctype_digit($value)) {
       $value = PhabricatorTime::parseLocalTime($value, $this->getViewer());
     }