Add null support to DataPrepperExpressions

[sbayer55]

Is your feature request related to a problem? Please describe.
Today if a JsonPointer results in a null value an exception is thrown. Using Data Prepper Expression Syntax null values cannot be hard coded. As of Data Prepper 1.3.0 all operators are not null safe.

Describe the solution you'd like
Add support for a null keyword.

Example:

/status_code in {null, 400, 404}

Describe alternatives you've considered (Optional)
Handling null values with exceptions.

Additional context
n/a

[kkondaka]

We will support equal and not-equal operators with "null" keyword initially. Support for "in" operator may be added later.

[dlvenable]

Closed by #1946