signon: optimize _signon_auto_bind() call

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

signon/middleware.py

@@ -88,7 +88,8 @@ class SignonMiddleware:
 
             # Try to auto_bind identities if needed.
             if getattr(settings, "SIGNON_AUTO_BIND", False):
-                self._signon_auto_bind(request)
+                if any(x.person is None for x in request.signon_identities.values()):
+                    self._signon_auto_bind(request)
 
             return