issue_484: Null pointer passed to DPLParserCatalystVisitor

[Abigael-JT]

• throw an exception when DPLParserCatalystContext is missing instead of passing a Null pointer to DPLParserCatalystVisitor
• return NullNode instead of null

[elliVM]

Change looks good. Some changes requested to improve code quality

[Abigael-JT]

Removed DPLParserCatalystContext null check, added ifInfoEnabled guard to logger messages with method call and removed commented code

[elliVM]

Found an issue in testing when NullNode was throwing an exception when cast to an ColumNode. Before this null value fix the null value would pass through as a null to visitLogicalStatementCatalyst() and return a NullStep, which would then result as empty dataset expected by the test.
To allow the NullNode pass through without exceptions added NullNode checks before casting in visitSearchTransformationRoot() which is not a pretty solution.

[51-code]

Seems good. I don't think there is a solution that can avoid instanceof calls for this.

[eemhu]

Would it be a good alternative to add isStub() or similar method to the Node interface, and NullNode would return true, otherwise false? This way it could be used instead of using instanceof checks?

[Abigael-JT]

added isStub() method to Node interface and rebased

[Abigael-JT]

Rebased

[Abigael-JT]

Rebased

Seems to be resolved