니 is NOT a shortened form of '니까'
~니 - formal impolite interrogative ending
~습니까/ㅂ니까 - formal polite interrogative ending
Both serve similar function (creating questions) and look somewhat similar, but are different nevertheless and are used in different contexts! A Korean person might be offended if you ask him or her using '니', since that would be simply rude in many cases. Saying that one of them is a shortened form of the other is an overstretch.