Validate empty Swagger property names

[king-407]

Fixes #2339.

What changed

This adds validation for Swagger 2.0 schemas that contain an empty property name under properties.

Previously, a schema like this was parsed without reporting a validation message:

"properties": {
  "AA": { "type": "string" },
  "": { "type": "number" }
}

The converter now adds a validation message for the empty property name while continuing to parse the document.

Testing

Ran the focused regression test and the full V2ConverterTest class:

mvn -pl modules/swagger-parser-v2-converter -am -Dtest=V2ConverterTest#testEmptyPropertyNameHasMessage -Dsurefire.failIfNoSpecifiedTests=false test
mvn -pl modules/swagger-parser-v2-converter -am -Dtest=V2ConverterTest -Dsurefire.failIfNoSpecifiedTests=false test

[ewaostrowska]

Thanks for the fix and the regression test!
We looked at the relevant specs and don't think an empty key in properties is a Swagger 2.0 validation error.

The OpenAPI 2.0 Schema Object applies JSON Schema semantics to properties. JSON Schema Draft 4 requires properties to be an object where each value is a valid schema, but says nothing about the names being non-empty. The official OpenAPI 2.0 validation schema has no such restriction either.

Duplicate names are a separate problem. RFC 8259 recommends unique names and warns that behavior is unpredictable otherwise, but by the time SwaggerConverter receives a Map<String, Property>, duplicates have already been collapsed. A converter traversal can't reliably detect that data loss.

Emitting a warning here would flag spec-valid documents as broken and could cause consumers that treat parser messages as errors to reject those documents.

We're closing this without merging. Thanks again for digging into this!