fix ServicePrincipalCertificate not reading from Secret

[nojnhuh]

What type of PR is this?
/kind bug

What this PR does / why we need it:

This change broke CAPZ's ability to read Service Principal certificate content from a Secret: https://github.com/kubernetes-sigs/cluster-api-provider-azure/pull/5200/files#diff-b6ab207506064afcbd1d777c1c7296e2953d2653a8843bfb39107af5cf5b0a87R213

I caught this while iterating on some of the unit tests that intersect with this. I'll try to refactor the tests to better catch things like this later.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

NONE

[nojnhuh]

/assign @willie-yao
fyi @bryan-cox

[nojnhuh]

/milestone v1.18

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.86%. Comparing base (4ff53b9) to head (3bd4c57).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5277   +/-   ##
=======================================
  Coverage   52.86%   52.86%           
=======================================
  Files         270      270           
  Lines       29121    29123    +2     
=======================================
+ Hits        15395    15397    +2     
  Misses      12933    12933           
  Partials      793      793           

☔ View full report in Codecov by Sentry.
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[bryan-cox]

/lgtm

[k8s-ci-robot]

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/lgtm

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[mboersma]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 6ef38c61231a95bc39aaa413e4bae9751750dbd8

[willie-yao]

/lgtm
/approve

[k8s-ci-robot]

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