Enable reading NULL_is_zero from symbol table

[tautschnig]

This was left commented out ever since introducing it in 5952f6a,
which is reasonable for it required bumping the goto-program version. We
have incremented the version since 2014, and thus it is safe to enable
this now.

• 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 #5681 (bb7c787) into develop (de7f873) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5681   +/-   ##
========================================
  Coverage    69.64%   69.64%           
========================================
  Files         1248     1248           
  Lines       100915   100915           
========================================
  Hits         70285    70285           
  Misses       30630    30630           

Flag	Coverage Δ
cproversmt2	43.36% <0.00%> (ø)
regression	66.62% <100.00%> (ø)
unit	32.18% <0.00%> (ø)

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

Impacted Files	Coverage Δ
src/util/config.cpp	57.51% <100.00%> (ø)

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[martin-cs]

OK but ... for those developers who weren't paying attention 7 years ago... any chance we could have some context or maybe a test (unit or regression) that this actually effects?