Support IS NULL and IS NOT NULL in the parser #21

Currently, the parser.rs (and planners) doesn't support IS NULL and IS NOT NULL operators. We would want to implement the support first in the parser, so later we could handle this scenario in the following steps of the query execution pipeline.

To achieve that, we may want to

• updates the BooleanExpression struct
• updates the comparison_expression method
• adds unit tests to cover the changes