Identifying a blank cell

Hi Experts,

I am trying to run the following UDV, but am not having any luck.

The idea is this:

When the sales stage gets changed to stage 3 AND U_QuoteIssuedDate is blank, then put the date in.

Can anyone assist?

select
case 
when $[OPR1.Step_Id] = '3' AND $[OPR1.U_QuoteIssuedDate] = ''
then getdate()
end

thanks!