inheritdoc works correctly only if a closest parent has actual description (methods)

[arogachev]

trafficstars

Extracted from #194. Reported by @neoacevedo.

What steps will reproduce the problem?

https://github.com/yiisoft/yii2-apidoc/issues/194#issuecomment-990093808

What's expected?

Replaced description from a parent method.

What do you get instead?

Error: Call to a member function getNamespaceAliases() on null

https://github.com/yiisoft/yii2-apidoc/blob/c13a69e5defffdd72ce63c9fcfe0e88fadb60e82/helpers/ApiMarkdownTrait.php#L149

Additional info

Q	A
Yii version	doesn't matter
Yii APIDoc version	master
PHP version	doesn't matter
Operating system	doesn't matter