1151925: Improved filtered listing output when results are empty.

Fixed a bug/oversight in the --list-enabled and --list-disabled options
which would print the "available repositories" header without any repos
in cases where all of the available repos have been filtered out. A
proper message will now be displayed instead.
Merge pull request #1039 from candlepin/crog/1151925

1151925: Improved filtered listing output when results are empty.