user_sieve_path(): never return "" -- always return a sensible path

[ksmurchison]

This should fix an issue where mailbox_open_sieve() via dav_reconstruct would try to write to root

[elliefm]

Do you know which commit introduced the bug, or which version was first affected? I'll want to backport this

[ksmurchison]

@elliefm This occurred as part of the UUID mailboxes work:

4ca9393#diff-09238fdff26df10fe2205827617598a66ea1c9246bbf420962ff439ca13e9271

[elliefm]

Thanks, that tells me 3.6 will need the fix too