Start to replace @var with assert()

While `@var` annotations are good for convincing static analyzers of
types, we could still encounter runtime warnings. Using assert() will
convert those warnings/notices into exceptions.

If folks like this approach I can apply it to the remaining classes in
managable chunks.

Iterator/MapReduce.php

@@ -185,10 +185,11 @@ protected function _execute(): void
             throw new LogicException('No reducer function was provided');
         }
 
-        /** @var callable $reducer */
         $reducer = $this->_reducer;
-        foreach ($this->_intermediate as $key => $list) {
-            $reducer($list, $key, $this);
+        if ($reducer !== null) {
+            foreach ($this->_intermediate as $key => $list) {
+                $reducer($list, $key, $this);
+            }
         }
         $this->_intermediate = [];
         $this->_executed = true;