Support byte-extracting empty structs

[tautschnig]

Empty structs are a GCC extension, which in general we do support.
Notably, boolbvt::convert_struct happily produces (empty) bitvectors for
empty structs. This commit makes sure byte-extracting empty structs is
equally supported.

• 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).
• 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

Merging #6653 (e021eef) into develop (4c2c27d) will increase coverage by 0.00%.
The diff coverage is 80.35%.

@@            Coverage Diff            @@
##           develop    #6653    +/-   ##
=========================================
  Coverage    76.73%   76.74%            
=========================================
  Files         1579     1579            
  Lines       182008   182171   +163     
=========================================
+ Hits        139671   139802   +131     
- Misses       42337    42369    +32     

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.h	100.00% <ø> (ø)
src/ansi-c/expr2c.cpp	65.94% <ø> (ø)
src/solvers/flattening/boolbv.h	62.50% <ø> (ø)
src/solvers/flattening/boolbv_byte_extract.cpp	83.11% <ø> (+0.83%)	⬆️
src/solvers/flattening/bv_utils.h	85.71% <ø> (ø)
src/solvers/flattening/boolbv_add_sub.cpp	48.33% <43.75%> (-3.06%)	⬇️
src/ansi-c/c_typecheck_expr.cpp	76.59% <83.87%> (+0.09%)	⬆️
src/solvers/flattening/bv_utils.cpp	79.61% <96.87%> (+1.14%)	⬆️
src/goto-programs/remove_vector.cpp	97.00% <100.00%> (+0.31%)	⬆️
src/solvers/flattening/boolbv.cpp	80.61% <100.00%> (+0.10%)	⬆️
... and 2 more

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