Fix NullReferenceException related to sorting/paging (#229)

The code that calls the GetParameterValues method throws
NullReferenceException because it expects a string[]
not null.

Fix #229

generators/server/templates/dotnetcore/src/JHipsterNet/Pagination/Extensions/QueryStringExtensions.cs.ejs

@@ -23,11 +23,12 @@ namespace JHipsterNet.Pagination.Extensions {
             var parameters = QueryHelpers.ParseQuery(query.ToString());
             return parameters.ContainsKey(name) ? parameters[name][0] : null;
         }
+
         public static string[] GetParameterValues(this QueryString query, string name)
         {
-            if (query == null || string.IsNullOrEmpty(name)) return null;
+            if (query == null || string.IsNullOrEmpty(name)) return new string[0];
             var parameters = QueryHelpers.ParseQuery(query.ToString());
-            return parameters.ContainsKey(name) ? parameters[name].ToArray() : null;
+            return parameters.ContainsKey(name) ? parameters[name].ToArray() :  new string[0];
         }
     }
 }