Bug 34184: Add empty option for suggestion "document type"

Bug 34184: Add empty option for suggestion "document type"

Currently when editing a suggestion, the document type will preselect to the
first entry as we don't have an empty entry.
This can lead to data errors easily.

Test plan:
Create a new suggestion
=> Notice that "document type" is empty by default, pick one
Edit it again
=> Notice that the value is kept and empty can be picked, pick empty
Edit again
=> Empty is selected and saved in DB

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>