[BUG][Python] Generated python client fails to correctly process optional response bodies

Created by: telday

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

Attempting to use optional request bodies in the Python generated client, then passing nothing in for the body kwarg, causes the client to crash and output:

Reason: Cannot prepare a request message for provided
                             arguments. Please check that your arguments match
                             declared content type.

This is because in the rest client the if statement choosing how to format the request body fails to account for a body = None

openapi-generator version

4.3.1 However the offending portion of the rest client generates identically in the latest version.

OpenAPI declaration file content or url

paths:
  '/test/':
    post:
      operationId: 'testing'
      requestBody:
        required: false
        content:
          text/plain:
            schema:
              type: string
      responses:
        "200":
          description: "The secret value was added successfully"

Steps to reproduce

Create a spec file with the above path in it, generate a python client, then attempt to make a request without passing the body kwarg