Add missing test for None in sqlalchemy query filter

In sqlalchemy a comparison against a non-null value will not match
null values, so the existing code at the end of
db.sqlalchemy.api.instance_get_all_by_filters() will not return
instances where vm_state is NULL.

This would be fine if the vm_state could never be null, but it
is declared as "nullable" in the Instance object. In many cases
"vm_state" will in fact have a value, but not all--in
get_test_instance() in test/utils.py the value of "vm_state" is not
specified.  There may be other similar cases.

Accordingly, this commit updates the test to explicitly check for None.

Without this fix the unit tests for bug 1292963 will fail because
the test instances have a vm_state of None.

Closes-Bug: 1294756
Related-Bug: 1292963
Change-Id: I271cff22dec160fd0e76abadefd0fe06d32c3227
Merge "Add missing test for None in sqlalchemy query filter"