Bug 16325: Do not return all suggestions if search for STATUS=''

Bug 16325: Do not return all suggestions if search for STATUS=''

This is a quick and dirty way to fix a bad bug in a messy area.
The "unknown status" tab in the suggestions table display all the
suggestions. It should only display suggestions with a STATUS=''

Test plan:
- Create some suggestions
- Go to Home > Acquisitions > Suggestions management
- Edit some suggestions and give them different status,
e.g. accepted, rejected, pending.
- Verify that they appear in the tabs as appropriate
- Edit one suggestion, set "Mark selected as" to --Choose a status--
=> Without this patch: New tab "Status unknown" containing all
suggestions
=> With this patch: tab contains only suggestions with "Unknown status"

Works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <bredan@bywatersolutions.com>
(cherry picked from commit 44e0cb2895f5e9fbf541603f5497f67f58fd5baf)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>