Use source_locationt::nil() instead of local static_cast

[tautschnig]

Cleanup, no change in behaviour: just use the existing (and previously
already used in multiple places) static nil() member of source_locationt
instead of repeating its implementation in several places.

• 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 #6640 (216e74e) into develop (c2cbaad) will increase coverage by 0.06%.
The diff coverage is 99.70%.

@@             Coverage Diff             @@
##           develop    #6640      +/-   ##
===========================================
+ Coverage    76.65%   76.72%   +0.06%     
===========================================
  Files         1579     1579              
  Lines       181612   181938     +326     
===========================================
+ Hits        139220   139587     +367     
+ Misses       42392    42351      -41     

Impacted Files	Coverage Δ
src/cpp/cpp_name.h	98.11% <0.00%> (ø)
src/goto-programs/goto_program.h	90.25% <ø> (-0.03%)	⬇️
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	52.67% <100.00%> (+15.75%)	⬆️
...solvers/smt2_incremental/smt_bit_vector_theory.cpp	100.00% <100.00%> (ø)
src/util/expr.cpp	73.33% <100.00%> (ø)
...t/solvers/smt2_incremental/convert_expr_to_smt.cpp	100.00% <100.00%> (ø)
...solvers/smt2_incremental/smt_bit_vector_theory.cpp	100.00% <100.00%> (ø)
src/util/std_expr.h	93.03% <0.00%> (+0.92%)	⬆️
src/util/floatbv_expr.h	72.30% <0.00%> (+2.30%)	⬆️

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