KeyValueStore: Remove assert for collection_getattr method

Merge 22b6c1a5317504e87042ffeff2b2d0d0b71cac5e introduce
5dc990c1ca821620993417ef4acc51a2b0a34606 commit which will lookup "remove"
attr. It shouldn't exist for most of cases but KeyValueStore will crash
if attr isn't exist. It seemed that this assert isn't correct.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
if out.size() is empty shouldn't we return ENODATA? a unit test for this would be helpful.. i'm not sure if the ENODATA should come from teh get_values_with_header helper above or not, but a test that does a zero-length attr, non-existent attr, and normal attr would quickly clear it up.