config-package-dev should always generate a "diverted-files" substvar, even if empty

[jdreed]

We encourage people to put the Conflicts/Provides? fields in when using c-p-d, but if we don't divert anything, dpkg-gencontrol generates a warning. I'd rather see people always putting those fields in, and leaving them empty if unneeded, vs encouraging people to remember to add them when necessary. So at a minimum, the dh7 version should generate the substvar regardless of whether something is diverted/hidden or not. (This assumes than an empty Conflicts/Provides? is not an error, and I don't think it is.)

Fixing #867 would render this irrelevant, since the only time you use c-p-d without diverting something is when you're just using it to undivert/unhide files.