mysql_variables: fix boolean value handling

[Andersson007]

SUMMARY

Fixes #652

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.36%. Comparing base (0bc3e3d) to head (b526a42).
Report is 4 commits behind head on main.

❗ Current head b526a42 differs from pull request most recent head 258eeba

Please upload reports for the commit 258eeba to get more accurate results.

Files	Patch %	Lines
plugins/modules/mysql_variables.py	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #653       +/-   ##
===========================================
+ Coverage   23.87%   72.36%   +48.48%     
===========================================
  Files          29       16       -13     
  Lines        2739     2573      -166     
  Branches      687      666       -21     
===========================================
+ Hits          654     1862     +1208     
+ Misses       2025      488     -1537     
- Partials       60      223      +163     

Flag	Coverage Δ
integration	72.28% <87.50%> (?)
sanity	?
units	?

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[Andersson007]

@laurent-indermuehle thanks for helping with this!