TestFunctionsForJSON: added new test method
testSET_FIELDFunctionWithNullValues, which tests the SET_FIELD (JSON)
function using null values (quoted and not), and related queries; also a
new version of (private) method testProcWithValidJSON, with the first
arg a String; and related Stored Procs NullSetFieldDocProc,
NullSetFieldProc and SelectSetFieldProc.