null or '' value filter on SDP table

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Say, we have 'Name','EmpID','CIty', 'State' columns in a SDP table.

filter/search input fields on the table are State, Name and EmpID. filter criterion operator on EmpID is set as '$eq'.

I have few records where EmpID value is null. when I try to retrieve these records using my search criterion on the table. I am unable to do so. I get 'No data to display' as my result. 

my REST query on network tab is sent as EMP ID = 'NULL' OR 'EMP ID' =' ' ' '

$pr operator ignores these records before table display.