tools/check_copyright: Return true if no changed file is detected

An edge case may happen when a patch only have renamed files and the
check_copyright.py script return error because there are no changed
files to check. This patch updates the script's run() return value
based on the following condition
-Return true if no changed file is detected.
-Return false if git command fail.

Signed-off-by: Wen Ping Teh <wenping.teh@arm.com>
Change-Id: Ie4ead662b5c51b53f4dff0a3742e8c8cfefb2577