rec: Don't validate a NXD with a NSEC proving that the name is an ENT

[rgacogne]

Short description

A NSEC from a.powerdns.com. to a.b.powerdns.com. proves that b.powerdns.com. exists and is an empty non terminal, so it cannot be used to prove that b.powerdns.com. does not exist.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[omoerbeek]

This makes sense to me though I'd like @Habbie to double check as well.