scripts/cpplint.py skips the include checks if module_dependencies.txt exists but is empty

[martin-cs]

CBMC version: 91a0307 on develop
Operating system: Debian
Exact command line resulting in the issue: touch modependencies.txt; scripts/cpplint.py
What behaviour did you expect: That all non-system headers were flagged as lint issues.
What happened instead: The inclusion check was disabled.

[martin-cs]

Further checking and I am confident this is a real issue.