Do not access all methods

Avoid doing a getattr() on all attributes, this is troublesome
for properties that are deprecated such as .results

Change-Id: Ic56d113aa913981d7941b0ae66e6d72e19af5242
Call is_empty() if available

To avoid a storm warning, use is_empty() if there's such a method
on a pass in instances list.

Change-Id: If673628f87e35342569d650478b9d01f509ba742