text: Title: Add Default of None for selected_options for ViewStateValue
  Body: Having a default of None will allow for consistency within the `ViewStateValue` object. All other attributes default to `None`, but `selected_options` does not get set if the input is an empty list (ex Checkbox). This results in an `AttributeError`.
  
  ### Reproducible in:
  
  #### The Slack SDK version
  
  `slack-bolt==1.4.4`
  
  #### Python runtime version
  
  `Python 3.8.3`
  
  #### OS info
  
  ```
  OS info
  ProductName:	Mac OS X
  ProductVersion:	10.15.7
  BuildVersion:	19H524
  Darwin Kernel Version 19.6.0: Tue Jan 12 22:13:05 PST 2021; root:xnu-6153.141.16~1/RELEASE_X86_64
  ```
  
  #### Steps to reproduce:
  
  (Share the commands to run, source code, and project settings (e.g., setup.py))
  
  1. Instantiate a `ViewStateValue` with `selected_options` as an empty list
  2. See below
  
  ### Expected result:
  
  `selected_options` should default to None
  
  ### Actual result:
  
  ```
  >>> d = {"selected_options": []}
  >>> v = ViewStateValue(**d)
  >>> v
  <slack_sdk.ViewStateValue>
  >>> v.selected_channel is None
  True
  >>> v.selected_options is None
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  AttributeError: 'ViewStateValue' object has no attribute 'selected_options'
  ```
  
  ### Requirements
  
  For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. :bow:
  
  Please read the [Contributing guidelines](https://github.com/slackapi/python-slack-sdk/blob/main/.github/contributing.md) and [Code of Conduct](https://slackhq.github.io/code-of-conduct) before creating this issue or pull request. By submitting, you are agreeing to those rules.
  