autodocs-ycp.ami: only call ycpdoc if YCP files are found

This way it works without error (producing no autodocs) if all YCP
files are converted to Ruby. We can reduce ycp-killer patches by
1122 lines.

The plan is to add autodocs for Ruby via Yard once the Ruby code is
in git.
Merge pull request #12 from mvidner/master

autodocs-ycp.ami: only call ycpdoc if YCP files are found