Fixed BasePageChooserPanel.render_js()

Addresses the "ForeignKey null=False" issue; new instances were throwing an exception, which prevented the edit handler from rendering the proper JavaScript in _editor_js.html.
Merge branch 'bpcp-fix' of https://github.com/rfosterslo/wagtail into rfosterslo-bpcp-fix
Add rfosterslo as contributor
Merge branch 'master' of github.com:torchbox/wagtail