cr_checker: do not add enforce copyright headers for empty files

[fbaeuerle]

With bazel it is quite common to have empty BUILD files for directories containing .bzl files, so we skip the copyright headers from those and all other empty files.

[4og]

Ideally, we need a unit test for it, but since the core functionality of the cr_checker is completely untested, I'm fine with merging this change. Here is the issue to add missing tests: #99