Fix null search query at the caller and add regression test

Coalesce a null `q` to an empty string in AbstractSearcher::search()
rather than in GambitManager::explode(), where the `string` parameter
type makes `?? ''` dead code that fails PHPStan. This keeps the gambit
chain's type contract intact and also covers an empty/absent `q`.

Adds a regression test asserting a null search query does not emit the
str_getcsv() null deprecation (fatal on strict PHP 8.1+ setups).

Author: imorland

framework/core/src/Search/AbstractSearcher.php

@@ -52,7 +52,7 @@ public function search(QueryCriteria $criteria, $limit = null, $offset = 0): Que
 
         $search = new SearchState($query->getQuery(), $actor);
 
-        $this->gambits->apply($search, $criteria->query['q']);
+        $this->gambits->apply($search, $criteria->query['q'] ?? '');
         $this->applySort($search, $criteria->sort, $criteria->sortIsDefault);
         $this->applyOffset($search, $offset);
         $this->applyLimit($search, $limit + 1);

framework/core/src/Search/GambitManager.php

@@ -64,7 +64,7 @@ public function apply(SearchState $search, $query)
      */
     protected function explode($query)
     {
-        return str_getcsv($query ?? '', ' ');
+        return str_getcsv($query, ' ');
     }
 
     /**

framework/core/tests/integration/api/users/ListTest.php

@@ -313,4 +313,33 @@ public function email_gambit_only_works_for_admin()
         $data = json_decode($response->getBody()->getContents(), true)['data'];
         $this->assertEquals([], Arr::pluck($data, 'id'));
     }
+
+    /**
+     * A search with a null query (rather than a string) must not trip the
+     * "passing null to a non-nullable parameter" deprecation on PHP 8.1+, which
+     * escalates to a fatal on stricter setups. Regression test for the gambit
+     * search choking on a null `q`.
+     *
+     * @test
+     */
+    public function search_with_null_query_does_not_emit_deprecation()
+    {
+        set_error_handler(function ($errno, $errstr) {
+            throw new \ErrorException($errstr, 0, $errno);
+        }, E_DEPRECATED);
+
+        try {
+            $response = $this->send(
+                $this->request('GET', '/api/users', [
+                    'authenticatedAs' => 1,
+                ])->withQueryParams([
+                    'filter' => ['q' => null],
+                ])
+            );
+        } finally {
+            restore_error_handler();
+        }
+
+        $this->assertEquals(200, $response->getStatusCode());
+    }
 }