export-data command fails to complete if --before-date isn't set

[cjg89]

Describe the bug

See title. In particular, the line below throws a ValueError due to attempting to filter Email results by None when --before-date isn't set.
https://github.com/UCF/PostMaster/blob/master/manager/management/commands/export-data.py#L193

Steps to Reproduce

1. Run python manage.py export-data --email="..."
2. Wait a while
3. Sadtimes

Expected Behavior

Because --before-date isn't a required param, I expect to be able to run the command without passing this param in, and have the command execute successfully.

Possible Fix

Set a default of the current date on self.before if --before-date is None?

Your environment:

n/a

Additional context

n/a

[jmbarne3]

We just need to make sure we check to make sure it has value before filtering by it, similar to what we do here: https://github.com/UCF/PostMaster/blob/master/manager/management/commands/export-data.py#L89-L92