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.

Reproduza a falha com o arquivo OpenAPI de espécies marinhas vinculado na issue, especialmente os schemas que contêm type: integer e default: "". Rastreie como os valores padrão de campos opcionais são tratados durante a geração do cliente e compare o comportamento com o tratamento indicado do swagger-parser. Considera-se concluído quando os endpoints afetados são gerados com sucesso e os valores padrão vazios são tratados como null.