Don't accept invalid value just because user insists

Change-Id: I0693f0e00226148faff2038cea320a506aabaea9
Correct error message: the name is the parameter's, not the column's

Change-Id: Ia26fb2dcd443f5c5b5cc48dff209d022388c7519
fdo#86852 correctly recognise NULL values in query parameter values dialog

Change-Id: I3c0cf976e0a9fbafe6eee768799a2f48c2ee0ea9