v:completed_item should always be a dict (not empty string), even before
the first completion.

Backported from vim_dev:
https://groups.google.com/forum/#!searchin/vim_dev/completeopt/vim_dev/tVsk0pdOGvs/fCzBbPkA4w0J

Use case:
https://github.com/Shougo/neocomplcache.vim/issues/426

Reviewed-by: Felipe Morales <hel.sheep@gmail.com>
Reviewed-by: Scott Prager <splinterofchaos@gmail.com>
Reviewed-by: Michael Reed <m.reed@mykolab.com>

Reviewed-by: Michael Reed <m.reed@mykolab.com>
Reviewed-by: Luke Andrew <luke.github@la.id.au>
Reviewed-by: Justin M. Keyes <justinkz@gmail.com>
Reviewed-by: Florian Walch <florian@fwalch.com>