text: Title: Support empty strings in enums
  Body: Hey there, thanks for the nifty tool!
  
  Something like
  
  ```yaml
          access_tier:
            type: string
            enum:
              - ""
              - "Archive"
              - "Hot"
              - "Cool"
  ```
  
  currently breaks openapi-python-client with
  
  ```
    File "openapi_python_client/parser/properties/__init__.py", line 545, in build_schemas
      schemas_or_err = update_schemas_with_data(name, data, schemas)
    File "openapi_python_client/parser/properties/__init__.py", line 521, in update_schemas_with_data
      prop, schemas = build_model_property(data=data, name=name, schemas=schemas, required=True, parent_name=None)
    File "openapi_python_client/parser/properties/__init__.py", line 261, in build_model_property
      prop, schemas = property_from_data(
    File "openapi_python_client/parser/properties/__init__.py", line 509, in property_from_data
      return _property_from_data(name=name, required=required, data=data, schemas=schemas, parent_name=parent_name)
    File "openapi_python_client/parser/properties/__init__.py", line 453, in _property_from_data
      return build_enum_property(
    File "openapi_python_client/parser/properties/__init__.py", line 343, in build_enum_property
      values = EnumProperty.values_from_list(enum)
    File "openapi_python_client/parser/properties/enum_property.py", line 62, in values_from_list
      if value[0].isalpha():
  TypeError: 'NoneType' object is not subscriptable
  ```
  
  While it doesn't seem like a very standard practice, I don't see anything in OpenAPI against it, and other code generators choose to support it (e.g. https://github.com/lukeautry/tsoa/pull/468).
  
  **OpenAPI Spec File**
  https://github.com/drakkan/sftpgo/blob/main/httpd/schema/openapi.yaml
  
  **Desktop (please complete the following information):**
   - OS: Linux
   - Python Version: 3.9
   - openapi-python-client version 0.8.0
  