Fix phpstan

Author: samsonasik

src/Collector.php

@@ -70,10 +70,12 @@ public function getResults(): array
         $count          = 0;
         $collectedData  = [];
         $isCallableWhen = is_callable($this->when);
+        $when           = $this->when;
 
         foreach ($this->data as $key => $datum) {
             if ($isCallableWhen) {
-                $isFound = ($this->when)($datum, $key);
+                /** @var callable(mixed $datum, int|string|null $key=): bool $when */
+                $isFound = ($when)($datum, $key);
 
                 Assert::boolean($isFound);