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 the failure with the marine species OpenAPI file linked in the issue, especially the schemas containing type: integer and default: "". Trace how optional-field defaults are handled during client generation and compare the behavior with the stated swagger-parser handling. Done means the affected endpoints generate successfully and empty defaults are treated as null.