Fix PHP 8.1 "strlen(null)" exceptions which block rendering the People page

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 general 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.

Use `phutil_nonempty_scalar()` instead in `PhabricatorSearchDateField.php` as
input could only be integer instead of string.

Closes T15297

Test Plan: Applied these four changes (on top of D25144, D25145, D25146) and `/people/` finally rendered in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15297

Differential Revision: https://we.phorge.it/D25147

src/applications/files/controller/PhabricatorFileDataController.php

@@ -29,7 +29,7 @@ final class PhabricatorFileDataController extends PhabricatorFileController {
     $request_kind = $request->getURIData('kind');
     $is_download = ($request_kind === 'download');
 
-    if (!strlen($alt) || $main_domain == $alt_domain) {
+    if (!phutil_nonempty_string($alt) || $main_domain == $alt_domain) {
       // No alternate domain.
       $should_redirect = false;
       $is_alternate_domain = false;

src/applications/search/field/PhabricatorSearchDateField.php

@@ -17,7 +17,7 @@ final class PhabricatorSearchDateField
   }
 
   protected function validateControlValue($value) {
-    if (!strlen($value)) {
+    if (!phutil_nonempty_scalar($value)) {
       return;
     }
 
@@ -32,7 +32,7 @@ final class PhabricatorSearchDateField
   }
 
   protected function parseDateTime($value) {
-    if (!strlen($value)) {
+    if (!phutil_nonempty_scalar($value)) {
       return null;
     }
 

src/view/layout/AphrontSideNavFilterView.php

@@ -92,12 +92,21 @@ final class AphrontSideNavFilterView extends AphrontView {
     return $this->getMenuView()->getItem($key);
   }
 
+  /**
+   * Add a thing in the menu
+   *
+   * @param string $key Internal name
+   * @param string $name Human name
+   * @param mixed $uri Destination URI. For example as string or as PhutilURI.
+   * @param string $type Item type. For example see PHUIListItemView constants.
+   * @param string $icon Icon name
+   */
   private function addThing($key, $name, $uri, $type, $icon = null) {
     $item = id(new PHUIListItemView())
       ->setName($name)
       ->setType($type);
 
-    if (strlen($icon)) {
+    if (phutil_nonempty_string($icon)) {
       $item->setIcon($icon);
     }