Fix a crash when a brand new file is added.

Checkstyle.py was not handling additions of new files resulting in
a crash. For new files git.modified_lines() returns None. Added code
handles that case.

Bug: 25852971
Change-Id: I91e2b8c92581ec9e89bcbbcd2d274f56c791f3a9
(cherry picked from commit 5b87dbad0cd8b5004c507b33060f2c093d72f98f)
1 file changed