Remove unqualified use of nullopt [blocks: #6749]

[tautschnig]

When moving to std::optional we'd have to use std::nullopt. Avoid doing
so by using {} instead, which works today and will work with
std::optional.

• 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 #6756 (7491954) into develop (c0f179c) will increase coverage by 0.02%.
The diff coverage is 88.11%.

@@             Coverage Diff             @@
##           develop    #6756      +/-   ##
===========================================
+ Coverage    76.87%   76.89%   +0.02%     
===========================================
  Files         1590     1590              
  Lines       183887   183973      +86     
===========================================
+ Hits        141364   141473     +109     
+ Misses       42523    42500      -23     

Impacted Files	Coverage Δ
src/ansi-c/goto_check_c.cpp	91.47% <ø> (-0.15%)	⬇️
src/memory-analyzer/gdb_api.h	100.00% <ø> (ø)
src/solvers/flattening/boolbv.h	62.50% <ø> (ø)
src/solvers/flattening/boolbv_quantifier.cpp	93.27% <50.00%> (+0.84%)	⬆️
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	72.09% <79.48%> (+3.07%)	⬆️
src/analyses/goto_rw.cpp	62.83% <100.00%> (+0.47%)	⬆️
src/ansi-c/expr2c.cpp	67.36% <100.00%> (+0.74%)	⬆️
src/goto-analyzer/unreachable_instructions.cpp	90.95% <100.00%> (ø)
src/goto-programs/builtin_functions.cpp	58.35% <100.00%> (+1.48%)	⬆️
src/util/edit_distance.cpp	100.00% <100.00%> (ø)
... and 13 more

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[tautschnig]

@chris-ryder I’ve got three approving reviews here, but strictly need yours as well. Could you please take a look?

[chris-ryder]

@tautschnig Apologies for the tardiness! :-)