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.

使用 issue 中链接的海洋物种 OpenAPI 文件复现该故障，尤其关注包含 type: integer 和 default: "" 的 schema。跟踪客户端生成过程中可选字段默认值的处理方式，并将其行为与所述的 swagger-parser 处理方式进行比较。当受影响的 endpoint 能够成功生成，并且空默认值被视为 null 时，即表示完成。