Signups: Do not bail if no key is found on the activation screen.

r9109 broke how the activation page is rendered if no key is passed in
the URL.  For example, if you go to example.com/activate/ instead of
example.com/activate/?key=XXX or example.com/activate/XXX/.

Anti-props r-a-y.

See #5831.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9203 cdf35c40-ae34-48e0-9cc9-0c9da1808c22