CONTRACTS: Allow NULL function pointer contracts

[remi-delmas-3000]

depends on #7328

Adds a third optional NULL parameter to the __CPROVER_requires_contract and __CPROVER_ensures_contract clauses. When specified, the meaning is that the function pointer in question can be either pointing to a function
satisfying the contract or be NULL.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Base: 78.28% // Head: 78.29% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (77592f7) compared to base (a295651).
Patch coverage: 79.28% of modified lines in pull request are covered.

❗ Current head 77592f7 differs from pull request most recent head d80ce8e. Consider uploading reports for the commit d80ce8e to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7327      +/-   ##
===========================================
+ Coverage    78.28%   78.29%   +0.01%     
===========================================
  Files         1642     1642              
  Lines       190004   190015      +11     
===========================================
+ Hits        148744   148773      +29     
+ Misses       41260    41242      -18     

Impacted Files	Coverage Δ
src/ansi-c/parser.y	81.09% <ø> (ø)
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
src/goto-programs/graphml_witness.cpp	52.94% <0.00%> (-0.15%)	⬇️
src/goto-programs/interpreter.cpp	52.38% <0.00%> (+0.17%)	⬆️
src/ansi-c/c_typecheck_code.cpp	78.88% <50.00%> (+1.51%)	⬆️
src/util/expr_initializer.cpp	85.10% <80.00%> (+1.77%)	⬆️
jbmc/src/java_bytecode/expr2java.cpp	86.25% <93.75%> (ø)
.../contracts/dynamic-frames/dfcc_wrapper_program.cpp	99.65% <97.61%> (+1.28%)	⬆️
src/ansi-c/c_expr.h	100.00% <100.00%> (ø)
src/ansi-c/c_typecheck_initializer.cpp	76.57% <100.00%> (+0.04%)	⬆️
... and 9 more

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[tautschnig]

#7328 is now merged, but various CI jobs appear to be failing.

[remi-delmas-3000]

#7328 is now merged, but various CI jobs appear to be failing.

this PR needs a rebase. I'm exploring an alternative format for these clauses so I'll leave this as-is for now

[remi-delmas-3000]

will be superseded by a new PR