fixed 2 issues

1) made usage of select2 field configurable: https://github.com/divio/djangocms-link/issues/8
2) select2 field raised an exception if value was `[]` (no page selected):

When using empty_values, you declare what values are accepted (no this was an empty list, meaning no empty values are accepted) ==> changing it to `empty_values = [[],]` would have fixed it.
Found out you can also declare only one empty_value => I found this more logical.