Set empty host when nil on finalize_from_uri

With ruby3.2, URI.parse now sets empty host instead of nil via:
dd5118f852

Adjust test case so, also with ruby <= 3.1, forcely set empty string for host
when nil to make finalize_from_uri behavior consistent between different
host ruby versions.

lib/vagrant-libvirt/config.rb

@@ -1304,8 +1304,8 @@ module VagrantPlugins
           end
         end
 
-        # Extract host values from uri if provided, otherwise nil
-        @host = uri.host
+        # Extract host values from uri if provided, otherwise set empty string
+        @host = uri.host || ""
         @port = uri.port
         # only override username if there is a value provided
         @username = nil if @username == UNSET_VALUE