Bug 19706: Fix bad json formed when there are no CCODE values

Bug 19706: Fix bad json formed when there are no CCODE values

To test:
1 - Delete all ccode values in your system
2 - Load the item search page
3 - View the console and note a js error
4 - Try to perform an item search to screen
5 - Internal server error ("unsupported format 'html'" in logs)
6 - Apply patch
7 - Reload page
8 - Note js error is gone
9 - Perform search to screen, success!

To test there is no regression:
1 - Have some values in CCODE authorised values
2 - Ensure collection drop down on item search displays correctly
3 - Ensure item search returns results ot screen with no js errors

Signed-off-by Owen Leonard <oleonard@myacpl.org>

Signed-off-by: David Bourgault <david.bourgault@inlibro.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>