Check that scheduler_disable_time is not None before re-enables
Check that scheduler_disable_time is not None before re-enables

Checking that scheduler_disable_time does not evaluate to False in a boolean
context is a bit unsafe, as I discovered when setting it to 0 in unit tests.
This checks whether it is None which is what was actually desired.
Houzz/luigi,Houzz/luigi,Houzz/luigi,Houzz/luigi