cpplint custom check: support empty module_dependencies.txt

[tautschnig]

We need to distinguish None from an empty list in our use of module dependencies.

Fixes: #4327

• 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

Base: 77.79% // Head: 78.25% // Increases project coverage by +0.45% 🎉

Coverage data is based on head (dfd2aa6) compared to base (f239925).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7304      +/-   ##
===========================================
+ Coverage    77.79%   78.25%   +0.45%     
===========================================
  Files         1568     1642      +74     
  Lines       180347   189749    +9402     
===========================================
+ Hits        140309   148487    +8178     
- Misses       40038    41262    +1224     

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv_index.cpp	72.91% <0.00%> (-18.06%)	⬇️
src/solvers/smt2/smt2_conv.h	84.61% <0.00%> (-15.39%)	⬇️
src/analyses/locals.h	85.71% <0.00%> (-14.29%)	⬇️
src/solvers/flattening/boolbv_byte_extract.cpp	69.73% <0.00%> (-13.39%)	⬇️
src/cpp/cpp_template_type.h	52.94% <0.00%> (-11.35%)	⬇️
src/util/replace_symbol.cpp	77.31% <0.00%> (-10.99%)	⬇️
src/ansi-c/ansi_c_typecheck.cpp	67.85% <0.00%> (-8.15%)	⬇️
src/util/invariant.cpp	72.54% <0.00%> (-7.85%)	⬇️
src/util/byte_operators.h	92.72% <0.00%> (-7.28%)	⬇️
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	83.04% <0.00%> (-5.29%)	⬇️
... and 402 more

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