Fix #670 by removing all None values from dict

[seratch]

Summary

This pull request fixes #670 by scanning None values from data, files, and params. There is no reason to keep them as they'll just result in TypeError.

After applying this change, the new sync mode WebClient in #662 also needs to be updated in the same manner.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #671 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #671      +/-   ##
==========================================
+ Coverage   87.17%   87.21%   +0.04%     
==========================================
  Files          15       15              
  Lines        1824     1830       +6     
  Branches      100      106       +6     
==========================================
+ Hits         1590     1596       +6     
  Misses        202      202              
  Partials       32       32              

Impacted Files	Coverage Δ
slack/web/base_client.py	80.86% <100.00%> (+1.05%)	⬆️

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[seratch]

This is safe enough. Let me merge this now.