Add IS NOT NULL to query builder

[katarinasupe]

Below should work:

Python: `match().node(variable='n').to().node(variable='m').where(item='n.name', operator=Operator.IS_NOT_NULL).return_()`
Cypher: `MATCH (n)-[]->(m) WHERE n.name IS NOT NULL RETURN *;`

Currently, there is no operator IS_NOT_NULL in Operator Enum and where() necessarily requires three arguments (literal or expression as third), while in this case there would be 2 args)