Fix logic for deciding when to write a default input body and content-type

[haydenbaker]

Description of changes:

• Fixes a bug where an operation with no input shape is treated with a default payload and therefore has the content-type header added to the request object:

// Serialized request object
HttpRequest {
  method: 'GET',
  ...
  headers: {
    'content-type': 'application/json',   <---- ** Should NOT be set **
    host: 'localhost:3001',
    'x-amz-user-agent': 'aws-sdk-js/0.0.1',
    'user-agent': 'aws-sdk-js/0.0.1 ua/2.0 os/darwin#23.4.0 lang/js md/nodejs#18.20.2',
    'amz-sdk-invocation-id': '32c12c5c-6d20-407b-bfec-b78c6b00a0cd',
    'amz-sdk-request': 'attempt=1; max=3'
  },
  body: '',
  ...
}

• This leads to a 415, UnsupportedMediaTypeException, when calling a server with said operation

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