An updated signal handler for the pre/post migrations signals inside

Django was assuming that the plan argument was not None, and was

crashing when we would emit those signals.
This change moves those signal emissions inside the nearby conditionals

so we only emit them if we're actually doing anything with migrations.
This also sets a default value for full_migration_plan so that static

checkers don't warn about it possibly being unbound.

Ran unit tests without causing a crash deep inside Django.