Fix PHP 8.1 "strlen(null)" exception rendering PHUISegmentBar without a label

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=df6c315ace5f), phorge(head=master, ref.master=3cc5ee6a33df)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/view/phui/PHUISegmentBarView.php:33]
```

Closes T15622

Test Plan:
After applying this change, page with the PHUISegmentBar without label renders as expected without an exception.

For example, visit this page:

/uiexample/view/PhabricatorAphrontBarUIExample/

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15622

Differential Revision: https://we.phorge.it/D25414

src/view/phui/PHUISegmentBarView.php

···
     require_celerity_resource('phui-segment-bar-view-css');
 
     $label = $this->label;
-    if (strlen($label)) {
+    if ($label !== null) {
       $label = phutil_tag(
         'div',
         array(