Fix string conversion for empty metadata instance objects

[ehennestad]

Fixed string conversion for empty objects to return empty string Add extra step in ControlledTerm to ensure scalar string "" is treated as empty

[github-actions]

Test Results (R2021b)

558 tests  ±0   558 ✅ ±0   1m 40s ⏱️ +9s
 16 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 1edd8d0. ± Comparison against base commit eca1a2e.

[github-actions]

Test Results (R2024b)

558 tests  ±0   558 ✅ ±0   1m 30s ⏱️ -4s
 16 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 1edd8d0. ± Comparison against base commit eca1a2e.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.58%. Comparing base (eca1a2e) to head (1edd8d0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...openminds/+internal/+mixin/CustomInstanceDisplay.m	50.00%	2 Missing ⚠️
...ode/internal/+openminds/+abstract/ControlledTerm.m	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
- Coverage   75.61%   75.58%   -0.03%     
==========================================
  Files         344      344              
  Lines        3691     3695       +4     
==========================================
+ Hits         2791     2793       +2     
- Misses        900      902       +2     

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