Add IS NOT NULL operator to query builder

[syzayd]

Closes #293.

What

Adds Operator.IS_NOT_NULL to the query builder, so property-existence filters can be expressed:

match().node(variable="n").to().node(variable="m").where(item="n.name", operator=Operator.IS_NOT_NULL).return_()

MATCH (n)-[]->(m) WHERE n.name IS NOT NULL RETURN *;

How

IS NOT NULL is the query builder's first unary (postfix) operator, so besides the enum entry, WhereConditionPartialQuery._build_where_query gains a small unary branch:

• unary operators require neither literal nor expression; the condition is rendered as item IS NOT NULL
• passing literal or expression with a unary operator raises GQLAlchemyExtraKeywordArguments, consistent with the existing "extra keyword arguments" semantics
• the unary set is a class constant (_UNARY_OPERATORS), so a future IS NULL is a two-line follow-up - happy to add it in this PR if you want the symmetric operator too

Tests and docs

• test_where_is_not_null, test_and_where_is_not_null (chaining), plus two error-path tests for literal/expression misuse - same style as the recently merged IN operator tests (Add IN operator to query builder #379)
• where() docstring updated and an IS NOT NULL section added to the query-builder how-to guide, mirroring the IN section
• ran pytest tests/query_builders/test_query_builders.py plus black --check and flake8 on the touched files locally - all green

Note

#293 shows a 2024 assignment; since the issue is still open with recent community interest and no linked PR, I hope a community contribution is welcome - happy to defer or adjust if work is already underway.

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