Handle reverse-lookup zones for unbound - fixes #4014

By default unbound returns nothing for private reverse lookups. Here is some information about that from https://www.unbound.net/documentation/unbound.conf.html
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       The  default  zones  are  localhost, reverse 127.0.0.1 and ::1, and the
       AS112 zones. The AS112 zones are reverse DNS zones for private use  and
       reserved IP addresses for which the servers on the internet cannot pro-
       vide correct answers. They are configured by default to  give  nxdomain
       (no  reverse  information)  answers.  The defaults can be turned off by
       specifying your own local-zone of that name, or using  the  'nodefault'
       type. Below is a list of the default zone contents.
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Just specifying 'nodefault' did not work. I found other threads where people used this in unbound.conf
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local-zone: "49.10.in-addr.arpa" typetransparent
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Note that it works if specifying the domain override with or without a final "." So the code here checks for the special cases of ".in-addr.arpa" and ".in-addr.arpa." at the end of a domain override name.

With this code my domain override entries for private reverse lookups work. I can traceroute around my internal network and the traceroute can show me the names that go with the private IP addresses at each hop.
What is the purpose of this check? Wouldn't be better to check if it's != "" ?