allow filtering by empty event end, registration start, registration …

[MegaphoneJon]

…end dates

Of the date fields on civicrm_event - start date, end date, registration start date, registration end date - only "start date" is required. However, Views can't search for empty dates without explicitly specifying a date can be empty. This adds that specification to the three event dates that aren't required.

[civibot]

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[eileenmcnaughton]

@jitendrapurohit @petednz - is this one you could review?

[jitendrapurohit]

BEFORE -

No filter to specify date = empty.

AFTER

Can be filtered with date=empty

The condition also works fine in rendering the results -

WHERE (( (civicrm_event.registration_start_date IS NULL ) AND (civicrm_event.end_date IS NOT NULL ) ))

[eileenmcnaughton]

Thanks for the speedy review @jitendrapurohit