Support using null-safe operator with null value

[willrowe]

This fixes the case when the MySQL null-safe operator <=> is used in a where condition with a RHS value of null. While the null-safe operator is already considered a valid operator, it is not currently considered valid by invalidOperatorAndValue when combined with a null value. This PR marks the combination as valid and expands the null convenience shortcut to convert it to is null/is not null .

I came across this when doing a comparison of a nullable column to a variable input which could also be null. I ended up using the null-safe operator so that the comparison would work as expected when the null values are encountered, since all other operators (=, !=, <>) return null instead of 0/1 when either side of the comparison is null.