Handle null fields comparisons in Rules processor

[priyank5485]

Currently, in Rules processor we allow users to create a rule. One of the conditions that users could could want is field <> null. Since it translates to storm sql under the hood we need to make sure that the rule translates to field is not null rather than field <> null so that it works correctly. Currently, it throws a runtime exception and we are unable to create such rules

[priyank5485]

Discussed this with @HeartSaVioR and seems that best way to address this is add two more dropdown items to dropdown in Rule creation dialog which says field is NULL and field is not NULL