Issue #5 - Fix for empty projects or code without modules

[matsoftware]

This PR fixes the issues related to scenarios where the project doesn't have any module or there are no files by gracefully failing without resulting in a nonzero error.

[codecov]

Codecov Report

Merging #6 into master will increase coverage by 0.51%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   96.06%   96.58%   +0.51%     
==========================================
  Files           6        6              
  Lines         458      468      +10     
==========================================
+ Hits          440      452      +12     
+ Misses         18       16       -2

Impacted Files	Coverage Δ
swift_code_metrics/_analyzer.py	99.27% <100%> (+0.8%)	⬆️
swift_code_metrics/_metrics.py	93.38% <100%> (+0.16%)	⬆️
swift_code_metrics/_helpers.py	100% <0%> (+2.38%)	⬆️

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