feat: Support IS NULL and IS NOT NULL conditions

[alireza-hosseini]

This PR Adds the possibility to check if the given field is null or not as a condition.
Here is an example of this new feature:

conditions = [
    {'field': 'trans_error',
     'type': 'FLOAT',
     'comparators': [
         {
             'condition': 'IS NOT NULL',
         }
     ]
     },
    {'field': 'rot_error',
     'type': 'FLOAT',
     'comparators': [
         {
             'condition': 'IS NOT NULL',
         }
     ]
     }
]