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
diff --git a/tools/plugins.py b/tools/plugins.py
index 3cc2b40..5f203a6 100644
--- a/tools/plugins.py
+++ b/tools/plugins.py
@@ -386,7 +386,8 @@
             emails = set()
             output.write(f"{m}: ")
             for (accounts, _) in self.maintainers.get(m):
-                emails.add(accounts.email)
+                if accounts.email:
+                    emails.add(accounts.email)
             output.write(f"{', '.join(sorted(emails))}\n")
 
     def render_page(self, output):