fix dns ui (#2229)

fix domains for nameserver only record, domain will keep after edit a
server with domains list

web/html/xui/dns_modal.html

@@ -53,7 +53,11 @@
               if (typeof dnsServer == 'object'){
                 this.dnsServer = dnsServer;
               } else {
-                this.dnsServer.address = dnsServer?? '';
+                this.dnsServer = {
+                  address: dnsServer ?? "",
+                  domains: [],
+                  queryStrategy: 'UseIP',
+                }
               }
             } else {
               this.dnsServer = {