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>
StoreTest: Add collection_getattr(s) tests

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
KeyValueStore: Return -ENODATA if collection_getattr is nonexist

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
KeyValueStore: Fix collection_getattrs problem when zero-length attr

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
MemStore: Return -ENODATA when collection_getattr hit nonexist attr

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
StoreTest: Clean up warning

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>