ensure log payload passed to MultilineLogger is a string

[zacharyburnett]

Closes #6407

This PR addresses the issue in #6407 where initializing matplotlib logging with usetex=True would cause the log message passed to the multiline logging handler to be bytes instead of the expected str

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[zacharyburnett]

regtests running at https://plwishmaster.stsci.edu:8081/blue/organizations/jenkins/RT%2FJWST-Developers-Pull-Requests/detail/JWST-Developers-Pull-Requests/758/pipeline

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 56.38%. Comparing base (6580914) to head (719f1bc).
Report is 2 commits behind head on master.

Files	Patch %	Lines
jwst/associations/lib/log_config.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7616      +/-   ##
==========================================
- Coverage   56.38%   56.38%   -0.01%     
==========================================
  Files         387      387              
  Lines       38716    38718       +2     
==========================================
+ Hits        21830    21831       +1     
- Misses      16886    16887       +1     

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