fix(occhab): made frontend works when OBSERVER_AS_TXT is true

[christophe-ramet]

fix #3787

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.48%. Comparing base (c9cdf35) to head (b32f822).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
..._module_occhab/backend/gn_module_occhab/schemas.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3788   +/-   ##
========================================
  Coverage    84.48%   84.48%           
========================================
  Files          133      133           
  Lines        10361    10361           
========================================
  Hits          8754     8754           
  Misses        1607     1607           

Flag	Coverage Δ
pytest	84.48% <60.00%> (ø)

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