Unify top factory getter implementations

The fact that the variables can be null should be documented in my
opinion.

The fact that they are null by default should be written down
explicitely, in my opinion.

The check should be done with "=== null", in my opinion, not with
"!" (it's not a bool) and not with "isset".

Change-Id: Ib5bb23de7b32b03706ea8126434b05b35efdf9da
Merge "Unify top factory getter implementations"