BUG: Error 500 when creating a promotion with unset dates

[rachidtt]

Reproduce:
Go to UI fill out all values except start and end (leave default "mm/dd/yyyy")
Press Create
=> Error 500

FIX:
We can either have the backend set default values for bad date inputs (ex: start = today, end=today+1month)
Or we can make the front end check that the dates are set, otherwise Create gives a message but doesn't send the request

Ideally, fix from the backend so that direct api calls (outisde of UI) don't throw the error