[5.5] Fix misleading behavior in Route::getAction()

[kamui545]

Patch for the following PR #20975

Pretty cool. However, am I the only one bothered by the fact that it is returning the action array if the key is not found?

Wouldn't it be more appropriate to return null instead as we expect a very specific property?

I find this misleading, especially if using it in a condition.
Eg: if ($route->getAction('controller')), even if there is no controllers it will return true...

As a bonus I added some tests.

[miclf]

For those who wonder, this PR covers all three cases with just a single line of code:

1. Without any key, it still returns the entire array (thus preserving the old behaviour).
2. If we give a key and if it exists, it returns the associated value.
3. If we give a key and if it does not exist, it returns null.