GraphML fixes to ensure SV-COMP compatibility

[tautschnig]

The witness linter exposed two bugs in our GraphML output. (See the individual commit messages.)

• 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 #5660 (50868ca) into develop (f85b11d) will increase coverage by 0.81%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #5660      +/-   ##
===========================================
+ Coverage    68.60%   69.42%   +0.81%     
===========================================
  Files         1243     1243              
  Lines       100621   100626       +5     
===========================================
+ Hits         69027    69855     +828     
+ Misses       31594    30771     -823     

Flag	Coverage Δ
cproversmt2	43.15% <100.00%> (?)
regression	66.31% <100.00%> (+<0.01%)	⬆️
unit	32.26% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/xmllang/graphml.cpp	68.66% <100.00%> (+0.68%)	⬆️
src/util/simplify_expr_int.cpp	80.08% <0.00%> (+0.27%)	⬆️
src/solvers/lowering/byte_operators.cpp	83.96% <0.00%> (+0.38%)	⬆️
src/cbmc/cbmc_parse_options.cpp	77.62% <0.00%> (+0.44%)	⬆️
src/util/mp_arith.cpp	48.59% <0.00%> (+0.70%)	⬆️
src/solvers/flattening/boolbv.cpp	69.09% <0.00%> (+0.90%)	⬆️
src/util/std_expr.h	89.90% <0.00%> (+1.10%)	⬆️
src/util/std_types.h	94.39% <0.00%> (+1.17%)	⬆️
src/solvers/smt2/smt2_solver.cpp	83.01% <0.00%> (+1.25%)	⬆️
src/util/pointer_offset_size.cpp	94.85% <0.00%> (+1.28%)	⬆️
... and 11 more

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