Strip null from query parameter schemas

[hniksic]

Fixes #274.

Query strings cannot express null values - a parameter is either present with a value or absent - so null types in the schema don't make sense and cause issues for SDK generators.

The transformation handles two cases:

• type: ["string", "null"], changed to type: "string"
• anyOf: [{$ref: ...}, {type: "null"}], changed to {$ref: ...}

This is enabled by default and can be disabled if needed:

aide::generate::strip_query_null_types(false);

A manual method TransformOpenApi::strip_null_from_query_params() is also available if you need to apply it at a specific point in your transform chain.

[jplatte]

Hey! Sorry for the very long wait time. Could you add a test for this new functionality?

[hniksic]

Hey, thanks for looking into it. I've added a test - please let me know if the location is good. (Another option was adding it to existing end-to-end tests in axum/mod.rs, but the functionality is not axum-specific.)