Making it easier to null-out the nightscout credentials

Author: loudnate

Loop/Models/NightscoutService.swift

@@ -47,7 +47,7 @@ struct NightscoutService: ServiceAuthentication {
     }
 
     var siteURL: NSURL? {
-        if let URLString = credentials[0].value {
+        if let URLString = credentials[0].value where !URLString.isEmpty {
             return NSURL(string: URLString)
         }