Fix PHP 8.1 "strlen(null)"/"strncmp(null)" exceptions in PhabricatorSearchDatasource typeahead

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.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/diffusion/typeahead/DiffusionSymbolDatasource.php:30]
```

```
ERROR 8192: strncmp(): Passing null to parameter #1 ($string1) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/phid/query/PhabricatorObjectQuery.php:41]
```

Closes T16162

Test Plan: Go to http://phorge.localhost/typeahead/class/PhabricatorSearchDatasource/

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16162

Differential Revision: https://we.phorge.it/D26170

src/applications/diffusion/typeahead/DiffusionSymbolDatasource.php

···
 
     $results = array();
 
-    if (strlen($raw_query)) {
+    if (phutil_nonempty_string($raw_query)) {
       $symbols = id(new DiffusionSymbolQuery())
         ->setViewer($viewer)
         ->setNamePrefix($raw_query)

src/applications/phid/query/PhabricatorObjectQuery.php

···
     $actually_phids = array();
     if ($names) {
       foreach ($names as $key => $name) {
-        if (!strncmp($name, 'PHID-', 5)) {
+        if (!phutil_nonempty_string($name) || !strncmp($name, 'PHID-', 5)) {
           $actually_phids[] = $name;
           $phids[] = $name;
           unset($names[$key]);