Fix PHP 8.1 "strlen(null)" exceptions searching in Conpherence

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/conpherence/query/ConpherenceFulltextQuery.php:58]
```

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/conpherence/query/ConpherenceFulltextQuery.php:73]
```

Closes T16062

Test Plan: Go to a Conpherence room and search for an existing string.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16062

Differential Revision: https://we.phorge.it/D25994

src/applications/conpherence/query/ConpherenceFulltextQuery.php

···
         $this->previousTransactionPHIDs);
     }
 
-    if (strlen($this->fulltext)) {
+    if (phutil_nonempty_string($this->fulltext)) {
       $compiler = PhabricatorSearchDocument::newQueryCompiler();
       $tokens = $compiler->newTokens($this->fulltext);
       $compiled_query = $compiler->compileQuery($tokens);
···
   }
 
   private function buildOrderByClause(AphrontDatabaseConnection $conn_r) {
-    if (strlen($this->fulltext)) {
+    if (phutil_nonempty_string($this->fulltext)) {
       return qsprintf(
         $conn_r,
         'ORDER BY MATCH(i.corpus) AGAINST (%s IN BOOLEAN MODE) DESC',