Fix callback with false value (#6)

Author: mcg-web

src/Generator/AbstractTypeGenerator.php

@@ -157,7 +157,7 @@ protected function processFromArray(array $values, $templatePrefix)
 
     protected function callableCallbackFromArrayValue(array $value, $key, $argDefinitions = null, $default = 'null', array $compilerNames = null)
     {
-        if (!isset($value[$key])) {
+        if (!$this->arrayKeyExistsAndIsNotNull($value, $key)) {
             return $default;
         }
 
@@ -259,6 +259,11 @@ protected function types2String(array $types)
         return '[' . implode(', ', $types) . ']';
     }
 
+    protected function arrayKeyExistsAndIsNotNull(array $value, $key)
+    {
+        return array_key_exists($key, $value) && null !== $value[$key];
+    }
+
     /**
      * Configs has the following structure:
      * <code>