Principal: be explicit about types of $this->pipeline

src/Principal.php

@@ -102,7 +102,7 @@ private static function apply(iterable $previous, callable $func): \Generator
      */
     public function filter(callable $func)
     {
-        if (!$this->pipeline) {
+        if (null === $this->pipeline || [] === $this->pipeline) {
             // No-op: either an empty array or null.
             return $this;
         }