Support NULL safe equals (<=>)

The NULL safe equals operator (<=>) gives a not supported error "Error Code: 122. IDB-1001: Function '<=>' can only be used in the outermost select or order by clause and cannot be used in conjunction with an aggregate function.".

This should be implemented. We could implement it by expanding it to:

a = b OR (a IS NULL AND b IS NULL)

As outlined in: https://mariadb.com/kb/en/mariadb/null-safe-equal/