pack/symm: Add test for missing symmsecret.upspinkey.
pack/symm: Add test for missing symmsecret.upspinkey.

Tidy up loose end from issue #75, checking that we properly notify
on empty password.  (Main problem of unpack not calling initAEAD
was fixed earlier.)

Change-Id: Ia022de21e61ec300dab2f22f4574afdd02ec5188
Reviewed-on: https://upspin-review.googlesource.com/5860
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin