[AIRFLOW-6246] Add support None in mysql_to_gcs

[stoynov96]

Make sure you have checked all steps below.

Jira

• My PR addresses the following AIRFLOW-6246 issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-6246

Description

• Here are some details about my PR, including screenshots of any UI changes:

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  test_convert_type_5

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[ashb]

Do you think you could add a unit test to cover this please?

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c43a5ec). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #6808   +/-   ##
=========================================
  Coverage          ?   84.23%           
=========================================
  Files             ?      676           
  Lines             ?    38312           
  Branches          ?        0           
=========================================
  Hits              ?    32274           
  Misses            ?     6038           
  Partials          ?        0

Impacted Files	Coverage Δ
airflow/operators/mysql_to_gcs.py	97.72% <100%> (ø)

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