Why is getGetPost() returning all data instead of null when index not found?

[tangix]

Noticed some issues with IncomingRequest::getGetPost() where I get all data instead of null when an index is not found. Seems to be related to #2839 but I can't say I understand the fix in #2860
The docs states that null should be returned if index not found.

[michalsn]

I believe it's already fixed in the dev branch via #3022.