Handle empty type in incremental SMT2 decision procedure

[tautschnig]

Skip symbols and nondet symbols with empty_typet (void) in gather_dependent_expressions to avoid attempting SMT sort conversion. Add early return in send_function_definition for empty-typed expressions as a defensive measure. Handle equality over empty type in set_to by treating it as vacuously true, matching the behavior of the non-incremental SMT2 backend.

Remove no-new-smt exclusion tag from havoc_slice_checks and memset_null regression tests since they now pass with the incremental SMT2 backend.

Fixes: #8077

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.41%. Comparing base (0170409) to head (cf62050).

Files with missing lines	Patch %	Lines
...ncremental/smt2_incremental_decision_procedure.cpp	80.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8907   +/-   ##
========================================
  Coverage    80.41%   80.41%           
========================================
  Files         1703     1703           
  Lines       188398   188407    +9     
  Branches        73       73           
========================================
+ Hits        151496   151506   +10     
+ Misses       36902    36901    -1     

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