Fix onehot0 flattening

[kroening]

This fixes the flattening of onehot0 expressions.

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 96.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.61%. Comparing base (fb28475) to head (26505a9).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/flattening/boolbv_onehot.cpp	60.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8536      +/-   ##
===========================================
+ Coverage    78.59%   78.61%   +0.01%     
===========================================
  Files         1728     1729       +1     
  Lines       199517   199564      +47     
  Branches     18315    18312       -3     
===========================================
+ Hits        156820   156887      +67     
+ Misses       42697    42677      -20     

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