Add message code for no variance zero null

[freddyaboulton]

Pull Request Description

Fixes #3371

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After creating the pull request: in order to pass the release_notes_updated check you will need to update the "Future Release" section of docs/source/release_notes.rst to include this pull request by adding :pr:123.

[codecov]

Codecov Report

Merging #3372 (23d1c38) into main (e431eff) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3372     +/-   ##
=======================================
+ Coverage   99.6%   99.6%   +0.1%     
=======================================
  Files        329     329             
  Lines      32229   32230      +1     
=======================================
+ Hits       32100   32101      +1     
  Misses       129     129             

Impacted Files	Coverage Δ
evalml/data_checks/no_variance_data_check.py	100.0% <ø> (ø)
evalml/tests/data_checks_tests/test_data_checks.py	100.0% <ø> (ø)
...s/data_checks_tests/test_no_variance_data_check.py	100.0% <ø> (ø)
evalml/data_checks/data_check_message_code.py	100.0% <100.0%> (ø)

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[bchen1116]

Nice! LGTM