feat: add IS NOT NULL operator to query builder

[LakshmiSravyaVedantham]

Summary

• Adds IS_NOT_NULL = "IS NOT NULL" to the Operator enum
• Updates _build_where_query() to handle unary operators that don't require a literal or expression argument
• Enables usage: where(item='n.name', operator=Operator.IS_NOT_NULL) → WHERE n.name IS NOT NULL

Closes #293

Test plan

• match().node(variable='n').to().node(variable='m').where(item='n.name', operator=Operator.IS_NOT_NULL).return_() should produce MATCH (n)-[]->(m) WHERE n.name IS NOT NULL RETURN *;
• Existing where() calls with literal/expression should continue working unchanged

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