Don't raise exception for existing but merely empty files

Fixes CHEF-4100

raise call where the file exists but is just empty causes trouble with its own code

It breaks its own insert_line_if_no_match() for no good reason (inserting a line in an empty file is a valid editing operation).

The only code caring about 'contents' is the private helper method search_match()
and 'contents' being a zero-length array there is no issue.