Fix TypeError for ManagedIdentityCredential when using service principal credentials

[M3GH4NN]

What does this PR do?

What issues does this PR fix or reference?

Fixes: #46

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 37.50000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 47.87%. Comparing base (49102d0) to head (2b06853).

❗ Current head 2b06853 differs from pull request most recent head 7b96fa6. Consider uploading reports for the commit 7b96fa6 to get more accurate results

Files	Patch %	Lines
src/saltext/azurerm/utils/azurerm.py	37.50%	4 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
- Coverage   50.04%   47.87%   -2.18%     
==========================================
  Files          19       19              
  Lines        1007     1013       +6     
  Branches       60       61       +1     
==========================================
- Hits          504      485      -19     
- Misses        492      515      +23     
- Partials       11       13       +2     

Flag	Coverage Δ
Linux	47.87% <37.50%> (-2.18%)	⬇️
project	64.32% <37.50%> (-7.19%)	⬇️
py310	47.77% <37.50%> (-2.28%)	⬇️
py38	47.72% <37.50%> (-2.28%)	⬇️
py39	47.82% <37.50%> (-2.18%)	⬇️
salt_3005_4	47.82% <37.50%> (-2.18%)	⬇️
salt_3006_4	47.87% <37.50%> (-2.18%)	⬇️
tests	44.53% <ø> (-1.31%)	⬇️

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[nicholasmhughes]

chore (blocking): looks good. just need a changelog file