Action Codegen: python-flask issue with optional inputs

# Issue with Hasura python-flask Codegen
I use a _Query_ action on Hasura. It queries a Flask REST server generated with Hasura in the Codegen tab. I would like my arguments to be optional, though that doesn't work as intended.

**E.g.** With this _action definition_
```GraphQL
type Query {
actionName (arg1: SampleInput): SampleOutput
}
```
I get 2 Python files from the Hasura Codegen feature, that are **actionName.py** and **actionNameTypes.py**. The latter is in my opinion the one to blame. It uses the _typing_ Python library to create a Python class structure that will resemble the structure from the JSON request.

In particular, I tried both to define the action with an _optional_ and a _mandatory_ **arg1** parameter. The only difference in the Python code is in the **Optional** keyword.

**Mandatory** version:
```Python
@dataclass
class actionNameArgs(RequestMixin):
arg1: SampleInput
```

**Optional** version:
```Python
@dataclass
class actionNameArgs(RequestMixin):
arg1: Optional[SampleInput]
```

This is though not enough to make _arg1_ actually optional, as stated [here (docs.python.org)](https://docs.python.org/3/library/typing.html#typing.Optional). A default value is in fact required.

**Example** of a working version:
```Python
@dataclass
class actionNameArgs(RequestMixin):
arg1: Optional[SampleInput] = None
```

This way, Python won't throw any Exceptions, since the **None** default value will be set to **arg1** when it will not be explicitly passed through the JSON call.

Do you think you can integrate this fix into the Python Codegen feature?

Start by locating the Python-Flask Codegen templates or entry points that generate actionNameTypes.py, then inspect how optional action arguments are rendered in the dataclass. Done means an optional arg1 receives a None default when omitted, with a regression test covering the generated output.

Written by the indexing model from the issue text.