Nullable Raw fields are not handled well for OAS 3.1.0

[tsokalski]

When converting a nullable Raw Marshmallow field into its corresponding OpenAPI representation for OAS 3.1.0, the generated representation does not match the behaviour of the actual schema/field:

import jsonschema
from marshmallow import Schema, fields

from apispec import APISpec
from apispec.ext.marshmallow import OpenAPIConverter


class TestSchema(Schema):
    raw_field = fields.Raw(allow_none=True)


spec = APISpec(title="Pets", version="0.1", openapi_version="3.1.0")
converter = OpenAPIConverter(
    openapi_version="3.1.0", schema_name_resolver=None, spec=spec
)
json_schema = converter.schema2jsonschema(TestSchema)
data = {"raw_field": "test"}
TestSchema().load(data)  # Works, as expected
jsonschema.validate(data, json_schema)  # Fails unexpectedly

The output of printing the json_schema variable above is the following:

{'type': 'object', 'properties': {'raw_field': {'type': ['null']}}}

It appears the FieldConverterMixin's field2nullable is appending null to the list of supported types for OAS 3.1.0, which makes sense in many cases, but when there's no pre-existing type value defined, the result is just ['null'], which means that the field can only be null. This doesn't match the behaviour of the defined schema.

I've implemented a workaround for this where I've overridden the DEFAULT_FIELD_MAPPING for marshmallow.fields.Raw to map to all supported JSON types (["boolean", "object", "array", "number", "string"]), but it's a bit of a hack, and would be nice if the library could support this a bit better.