split up the nil? and empty? cases, because they are different

situations

app.rb

@@ -50,9 +50,14 @@ class App < Roda
         end
 
         url = r.params['url']
-        if url.nil? or url.empty? 
+        if url.nil?
           response.status = 400
-          return {message: "Please enter a valid URL"}
+          return {message: "missing url parameter"}.to_json
+        end
+
+        if url.empty? 
+          response.status = 400
+          return {message: "invalid url parameter"}.to_json
         end
 
         if links.filter(:url => url).first.nil?