PHP 8.5: Search: Do not pass a null function key to results array

Summary:
Setting null as an array key is deprecated since PHP 8.5 per https://www.php.net/releases/8.5/en.php: "Using null as an array offset or when calling array_key_exists() is now deprecated. Use an empty string instead."

```
ERROR 8192: Using null as an array offset is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/applications/search/compiler/PhutilSearchQueryCompiler.php:469]
```

Closes T16401

Test Plan: Go to http://phorge.localhost/search/query/advanced/; enter "foo" in the "Query" field

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

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

Maniphest Tasks: T16401

Differential Revision: https://we.phorge.it/D26597

src/applications/search/compiler/PhutilSearchQueryCompiler.php

···
       $present_tokens = array();
       $absent_tokens = array();
       foreach ($results as $result) {
+        if (!isset($result['function'])) {
+          continue;
+        }
         $function = $result['function'];
 
         if ($result['operator'] === self::OPERATOR_ABSENT) {