Merge branch 'fix-nil-handling' into 'master'

Handle empty arguments case

`JSON.parse('')` results in an error, but `Oj.load('')` returns `nil`.

See merge request !6

app.rb

@@ -24,7 +24,7 @@ class MarketoTools < Sinatra::Base
       params = {}
     end
 
-    if params['event_name'] == 'user_create'
+    if params && params['event_name'] == 'user_create'
       user = User.new(params)
 
       client.createupdate_leads([user.to_param], lookup_field: :email)