SMT2: concatenations with zero-width operands

[kroening]

This enables support for concatenation expressions that have some zero-width operands in the SMT2 backend.

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
• 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

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.00%. Comparing base (05c2649) to head (add149c).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	83.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8829      +/-   ##
===========================================
- Coverage    80.00%   80.00%   -0.01%     
===========================================
  Files         1700     1700              
  Lines       188332   188342      +10     
  Branches        73       73              
===========================================
+ Hits        150679   150686       +7     
- Misses       37653    37656       +3     

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