Since UNIONs that would allow a column to be masked are
rejected, there is no need to check that masking is performed
since we will never get that far.
The masking filter now has a default fill value, so that if a
provided value does not match, then "X" is used.
The tests were modified to use "Y" as an explicitly provided
fill value (to distinguish from the default "X") and the results
were the default fill value would kick in were modified accordingly.
