Fix QFile::copy warning when the colors file doesn't exist

Summary:
There is no need to even look for a "Default.colors" file when
using the default color scheme, so skip the entire file copying
in that case.

Test Plan:
Just checked that the warning went away, I didn't test
the kde4 interaction.

Reviewers: mart, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8266