fix: PHP mixed type is never nullable

The PHP "mixed" type is a union type including null, see: https://www.php.net/manual/en/language.types.declarations.php#language.types.declarations.mixed

Therefore the type is never nullable, e.g. `?mixed $someParameter` is invalid and will generate a Fatal PHP Error: "Mixed types cannot be nullable, null is already part of the mixed type."

This commit will fix mixed parameters.

Author: pmzandbergen

lib/internal/Magento/Framework/Code/Generator/EntityAbstract.php

@@ -352,7 +352,7 @@ private function extractParameterType(
             $typeName = $parameterType;
         }
 
-        if ($parameter->allowsNull()) {
+        if ($parameter->allowsNull() && $typeName !== 'mixed') {
             $typeName = '?' . $typeName;
         }