Fix PHP 8.1 "strlen(null)" exception setting Story Points on a Task

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. Here we adopt `phutil_string_cast()` to
reply to the question "is this an empty string?".

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_string_cast() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

Closes T15390

Test Plan:
Enable `maniphest.points` in settings, for example with:

```
./bin/config set maniphest.points --stdin <<< '{"enabled":true}'
```

Then try to create a Task: it does not explode anymore in PHP 8.1+.

Also try to set various possible values from the Conduit API method "maniphest.edit".

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15390

Differential Revision: https://we.phorge.it/D25222

src/applications/maniphest/xaction/ManiphestTaskPointsTransaction.php

@@ -97,8 +97,17 @@ final class ManiphestTaskPointsTransaction
     return 'fa-calculator';
   }
 
+  /**
+   * Normalize your Story Points from generic stuff to double or null.
+   * @param  mixed $value Your raw Story Points
+   * @return double|null
+   */
   private function getValueForPoints($value) {
-    if (!strlen($value)) {
+    // The Point can be various types also thanks to Conduit API
+    // like integers, floats, null, and strings of course.
+    // Everything meaningful must be printable as a string.
+    $is_empty = phutil_string_cast($value) === '';
+    if ($is_empty) {
       $value = null;
     }
     if ($value !== null) {