Fix crash on multipart body referencing a schema without properties

[N39r19]

What

Fixes #466

swagger_parser crashes (aborting the whole generation) when an endpoint declares a multipart/form-data request body that references (via \$ref) an object schema without a properties field.

type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast at OpenApiParser.parseRestClients.parametersV3.

Root cause

The multipart \$ref branch assumes the referenced schema always has properties, while the inline (non-\$ref) branch already tolerates a missing properties map (properties = {}). This makes the \$ref branch consistent with the inline one.

Reproducer (real-world)

An ASP.NET Core / FastEndpoints file-import endpoint whose request DTO is an empty placeholder while the file is read from the stream emits a multipart body referencing an empty object schema ({ "type": "object" }), e.g. POST /api/admin/team-schedule/import → EmptyRequest. Today this wipes the entire generated client because make gen removes lib/src first and swagger_parser exits 0 despite the error.

Changes

• open_api_parser.dart: null-guard properties in the multipart \$ref branch (treat as empty form, like the inline branch).
• Added e2e test multipart_request_with_ref_no_properties.
• CHANGELOG entry.

Checklist

• dart format .
• dart analyze — no issues
• dart test — all tests pass (423 passed, 2 pre-existing skips)