openapi3filter: prefer non-empty value for repeated scalar query params

[sonnemusk]

What

When a scalar query parameter is repeated and the first value is empty, decode the first non-empty occurrence instead of always taking values[0].

Why

With allowEmptyValue: true, an empty first value short-circuits schema validation. That made order-dependent results:

• ?dateOfBirth=&dateOfBirth=not-a-date was accepted
• ?dateOfBirth=not-a-date&dateOfBirth= was rejected

A single empty value still works as before (#1228 / #1134).

Fixes #1230.

Test

• TestIssue1230
• existing TestIssue1134

[sonnemusk]

Thanks for the review.

• Dropped the verbose comment.
• On allowEmptyValue: DecodePrimitive only receives the schema, not the parameter, so it cannot gate on parameter.AllowEmptyValue without a wider API change. Preferring the first non-empty repeated value is the right fix for openapi3filter: repeated query param with empty first value bypasses schema validation #1230 either way: with allowEmptyValue: true, a leading "" would otherwise skip schema checks entirely (validate_request.go) and hide a later invalid value; with allowEmptyValue: false, an empty scalar is still validated by the schema when no non-empty sibling exists. Happy to follow up if you want allowEmptyValue threaded into the decoder interface.