Editing a discount from the staff dashboard fails if Activation Date is not set

[jkachel]

Something appears to be trying to format a blank date, and it results in "Invalid Date" being sent as part of the request payload. Because of this, saving fails - "Invalid Date" is, unsurprisingly, an invalid date and it fails the validation in the serializer.

This was tested with Firefox on macOS (113.0.1) but this is unlikely to be browser specific.

Steps to Reproduce

1. Open the staff dashboard and navigate to the Discounts area.
2. Edit a discount (ideally one that has an expiration date but not an activation date set).
3. Attempt to save the discount. Do not change the activation date.

Expected Behavior

The discount code saves successfully.

Actual Behavior

You get a 400 error, and the discount is not saved.

Related Issues

First encountered in https://github.com/mitodl/hq/issues/1498

Screenshot or Screencast

[pdpinch]

I'm not sure why this ancient issue just surfaced, but I think it will be moot as soon as we move to Universal ecommerce (if it isn't moot already)