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 中連結的海洋物種 OpenAPI 檔案重現此故障，特別關注包含 type: integer 和 default: "" 的 schema。追蹤用戶端產生過程中選用欄位預設值的處理方式，並將其行為與所述的 swagger-parser 處理方式進行比較。當受影響的 endpoint 能夠成功產生，且空的預設值被視為 null 時，即表示完成。