Handle "empty" default values as null

Describe the bug
"empty" default values for optional fields throw an error. swagger-parser explicitly allows these (even though they are technically out-of-spec) by converting them silently to null. So, there exist openapi json in the wild built by swagger tools that pass swagger validation, but throw an error here in openapi-python-client.

OpenAPI Spec File
https://www.marinespecies.org/rest/api-docs/openapi.yaml

grep for:

                schema:
                    type: integer
                    default: ""

There are three endpoints that have this property. One of which is AphiaTaxonRanksByID. It might be useful to check this endpoint out in the swagger docs here.

Desktop (please complete the following information):

• openapi-python-client version 0.28.0

Additional context
I am in contact with the folks at WORMS to try and find out how their openapi.yaml got this way. Will update here when I get more info.

Reproduce el fallo con el archivo OpenAPI de especies marinas enlazado en la issue, especialmente los esquemas que contienen type: integer y default: "". Rastrea cómo se gestionan los valores predeterminados de los campos opcionales durante la generación del cliente y compara el comportamiento con el tratamiento indicado de swagger-parser. Se considera terminado cuando los endpoints afectados se generan correctamente y los valores predeterminados vacíos se tratan como null.