Fix PHPStan error

Author: simonhammes

src/Prometheus/Storage/APCng.php

@@ -438,11 +438,10 @@ private function buildPermutationTree(array $labelValues): \Generator /** @phpst
         if (count($labelValues) > 0) {
             $lastIndex = array_key_last($labelValues);
             $currentValue = array_pop($labelValues);
-            if ($currentValue !== null) {
-                foreach ($this->buildPermutationTree($labelValues) as $prefix) {
-                    foreach ($currentValue as $value) {
-                        yield $prefix + [$lastIndex => $value];
-                    }
+
+            foreach ($this->buildPermutationTree($labelValues) as $prefix) {
+                foreach ($currentValue as $value) {
+                    yield $prefix + [$lastIndex => $value];
                 }
             }
         } else {