dnsdist: Fix getEDNSOptions() for {AN,NS}COUNT != 0 and ARCOUNT = 0

dnsdist: Fix getEDNSOptions() for {AN,NS}COUNT != 0 and ARCOUNT = 0

Since 1.5.0, calling getEDNSOptions() from Lua would result in a
ServFail for queries that had no records in additional but at least
one record in either the answer or authority section, such as a
NOTIFY, because of a bug in parseEDNSOptions(). That last function
incorrectly called slowParseEDNSOptions() in that case, triggering
an exception to be raised because slowParseEDNSOptions() does not
expect to be called for a packet with no record in the additional
section.
parseEDNSOptions() now returns `false` for packets that have no
record in the additional section.