Allow 0 for Mdm::WebVuln#confidence

[#45816713]

I misread
https://github.com/rapid7/metasploit-framework/blob/master/lib/msf/core/db.rb#L2414
and thought confidence had to be > 0 instead of it could not be < 0, so
I had made confidence range 1 .. 100 instead of the correct 0 .. 100.
Merge pull request #12 from rapid7/bug/0-confidence

Allow 0 for Mdm::WebVuln#confidence