Add more robust conversion of nwbinspector values to string

[ehennestad]

Motivation

Fixes a bug where conversion of NWB inspector metadata to a MATLAB structure fails when metadata is missing (values is represented as a py.NoneType)

How to test the behavior?

N/A

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[ehennestad]

Tests are currently failing due to this pending PR:
NeurodataWithoutBorders/nwbinspector#660

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.49%. Comparing base (08da374) to head (57eb0c8).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
inspectNwbFile.m	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #781      +/-   ##
==========================================
- Coverage   95.50%   95.49%   -0.01%     
==========================================
  Files         191      191              
  Lines        6922     6931       +9     
==========================================
+ Hits         6611     6619       +8     
- Misses        311      312       +1     

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