Fix NPE while decoding authority section of a DNS response

Motivation:

NullPointerException is raised when a DNS response conrains a resource
record whose NAME is empty, which is the case for the authority section.

Modification:

Allow an empty name for DnsEntry. Only disallow an empty name for
DnsQuestion.

Result:

Fixes #2686
Raise a meaningful exception instead of NPE

Motivation:

When decoding the NAME field in a DNS Resource Record, DnsResponseDecoder
can raise a NullPointerException if the NAME field contains a loop.

Modification:

Instead of raising an NPE, raise CorruptedFrameException so that the
exception itself has meaning.

Result:

Less confusing when a malformed DNS RR is received