Enable contribution views to filter on empty Receipt and Thank You dates

[aydun]

Adding 'allow empty' => TRUE to filter definitions for dates enables the is empty / is not empty filter options.

Useful if you want a view showing contributions that have/have not been thanked.

Before: A view filter on Thank You date or Receipt Date does not offer empty/not empty

After: A view filter on Thank You date or Receipt Date does offer empty/not empty

[civibot]

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[seamuslee001]

This makes sense to me and seems fine code wise

[seamuslee001]

I have tested this locally and confirms it works merging

[aydun]

Thanks @seamuslee001 !