Fix series.project migration logic

Fix series.project migration logic

The migration logic seems slightly off for 2 reasons:

1) If you already have series objects in your database, then adding a
non-nullable foreign key to projects will fail.

2) This may just be a remnant of my development database, but I had
series entries with no patches.

I think part #1 is *required*. I'm not totally sure about #2, but it
seems safe.

Signed-off-by: Andy Doan <andy.doan@linaro.org>
Reviewed-by: Stephen Finucane <stephen@that.guru>