_ContainerFromDicts: handle None source

When _ContainerFromDicts is called on some element which doesn't exist
in the config, because it is yet to be upgraded, it will receive its
value as None. We take care of this case by using an empty element of
the required target type.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>