Do not put null emails in plugins list

The generation of the list of plugins and associated maintainers
was failing because of a null e-mail, which may happen for
deleted accounts.

Avoid putting null e-mails in the rendered list and resume
the normal plugins page generation.

Bug: Issue 14180
Change-Id: I6065d5edc52792a6d2bfb7e6bad0f1a62ac91125
1 file changed