Clean up Union/Mixed check (#7339)

Author: samsonasik

rules/TypeDeclaration/NodeAnalyzer/CallTypesResolver.php

@@ -132,10 +132,6 @@ private function unionToSingleType(array $staticTypesByArgumentPosition, bool $r
                 $unionedType
             );
 
-            if ($staticTypeByArgumentPosition[$position]->isNull()->yes()) {
-                $staticTypeByArgumentPosition[$position] = new MixedType();
-            }
-
             if ($staticTypeByArgumentPosition[$position] instanceof UnionType) {
                 foreach ($staticTypeByArgumentPosition[$position]->getTypes() as $subType) {
                     if ($subType instanceof MixedType) {