Fix receive_date error saving on Edit Participant with Record Contribution

[larssandergreen]

Overview

@JoeMurray describes how to reproduce this issue here:

1. Backoffice register a contact in fall fundraiser, making sure to unselect Record Payment.
2. Edit the registration, enable Record Payment, Save.
3. On dmaster, it saves.
4. On test instance, it hangs (I got the same locally on master)

The issue was that receive_date was an empty string, so the check this PR replaces wasn't changing it to a default date. In some cases, that causes a DB error. On dmaster, it creates the contribution and transaction with an empty date. Not sure why the behaviour is different, but that doesn't really matter, because it is broken either way.

Before

Can't save Participant or Contribution created with empty receive_date if no receive_date entered on the form.

After

Participant saved as normal, Contribution created with current date and time or as entered on form.

[civibot]

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