Handle exception when analyzing an empty repository

[IP1102]

It might seem obvious to run this tool on a non-empty repository, but for few use cases like integrating this tool as part of an automated pipeline to analyze several repositories there might be few empty repositories. When ran on an empty repository, it fails to iter_commits and throws the following error:

git.exc.GitCommandError: Cmd('git') failed due to: exit code(128)
cmdline: git rev-list --reverse HEAD --
stderr: 'fatal: bad revision 'HEAD'

To fix this, I followed the same approach adapted in get_commits_modified_file method to handle empty modifications by checking GitCommandError exception (ref). I have added the test case and added it's corresponding repository in the .zip file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.26%. Comparing base (aed5f5b) to head (f4385e0).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
+ Coverage   97.25%   97.26%   +0.01%     
==========================================
  Files          17       17              
  Lines        1091     1096       +5     
==========================================
+ Hits         1061     1066       +5     
  Misses         30       30              

Files with missing lines	Coverage Δ
pydriller/git.py	97.12% <100.00%> (+0.10%)	⬆️

[ishepard]

LGTM! Thanks for the PR!