Fix passing null to strlen

$filter can be null as it's the default value passed in
ContactsMenuController.

On PHP 8.1 : strlen(): Passing null to parameter #1 ($string) of type string is deprecated

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Author: tcitworld

lib/private/Contacts/ContactsMenu/Manager.php

@@ -66,7 +66,7 @@ public function getEntries(IUser $user, $filter) {
 		$maxAutocompleteResults = max(0, $this->config->getSystemValueInt('sharing.maxAutocompleteResults', Constants::SHARING_MAX_AUTOCOMPLETE_RESULTS_DEFAULT));
 		$minSearchStringLength = $this->config->getSystemValueInt('sharing.minSearchStringLength', 0);
 		$topEntries = [];
-		if (strlen($filter) >= $minSearchStringLength) {
+		if ($filter && strlen($filter) >= $minSearchStringLength) {
 			$entries = $this->store->getContacts($user, $filter, $maxAutocompleteResults);
 
 			$sortedEntries = $this->sortEntries($entries);