Support IS NULL and IS NOT NULL in the datafusion planner #43

Currently, IS NULL and IS NOT NULL have been supported in the parser.rs, but not in the datafusion planner yet. We'll need to support the end-to-end query processing of these two keywords via the datafusion.

We may want to

• update the to_df_boolean_expr in the datafusion_planner/expression.rs
• add necessary integration tests in the test_datafusion_pipeline.rs
• update other code if necessary