bitbake: data_smart: Add explict None checks

bitbake: data_smart: Add explict None checks

Simple if xxx checks end up calling len(xxx). We're interested in the specific case
of None which means we can break out the iterator much earlier after the first
item. This adds in the specific tests for None in what is a hot path in the
data store code which gives small performance gains.

(Bitbake rev: a4d81e44a7cd3dafb0bf12f7cac5ff511db18e60)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>