Add is empty filter to search / api

Add is empty filter to search / api

This is already offered in Query
https://github.com/civicrm/civicrm-core/blob/5db2212e2d408f4611439734db1a31ab32dced2f/CRM/Contact/BAO/Query.php#L3420-L3428

And in Report
https://github.com/civicrm/civicrm-core/blob/c3fffe27cb8203634c7a2c047686ba3d12cc38bd/CRM/Report/Form.php#L2105-L2124

(the latter munges it in with NULL but as we often save empty strings NULL
does not alwasy work for strings)