Non accurate generation of stubs for methods which may return object or null

[sergeyklay]

For the following Zephir code:

/**
 * Returns the internal event manager
 */
public function getInternalEventsManager() -> <ManagerInterface> | null
{
    return this->eventsManager;
}

The following sub was generated:

/**
 * Returns the internal event manager
 *
 * @return mixed
 */
public function getInternalEventsManager(): ?ManagerInterface
{
}

It is a bit non accurate and should be as follows:

/**
 * Returns the internal event manager
 *
-* @return mixed
+* @return ManagerInterface|null
 */
public function getInternalEventsManager(): ?ManagerInterface
{
}

[sergeyklay]

Implemented in development branch