md-colors should repspond better to {} and null

[sdesmond]

I would expect md-colors="{}" and md-colors="null" to leave the element as though md-colors wasn't on the element. What I'm seeing is md-colors="null" throws an error, and md-colors="{}" leaves the element in its last state.

See the Pen for an example (open the console to see the error).
http://codepen.io/anon/pen/BKVZGo?editors=1011

Material Version 1.1.0-rc4

[sdesmond]

Thanks @EladBezalel.