Assertion for nil MOC, take 2

Okay, this should satisfy standard use cases of:
 1. Non-faulted MO with context
 1. Faulted MO without context (will throw assertion)
 1. Non-faulted MO without context (will work a-okay)

And, to be sure, I added a test to cover the use case for non-faulted/missing MOC.