Fix empty query parameter schema validation

[rootsec1]

An empty query value is rejected before schema validation whenever allowEmptyValue is omitted. This prevents schemas that explicitly allow an empty string, including enum values, from taking effect.

The empty-value error is now raised only when allowEmptyValue is explicitly false. When the deprecated field is absent, the schema decides whether the empty string is valid; explicit false behavior and warnings remain unchanged.

Tests cover validation and unmarshalling across OpenAPI 3.0, 3.1, and 3.2, including schema-allowed and rejected empty strings, explicit false, and omitted optional parameters.

Closes #1210