Bug 21398: Make search field mandatory when adding to a basket from an existing record

Bug 21398: Make search field mandatory when adding to a basket from an existing record

Test plan:
- Create a basket
- Click the "Add to basket" button
- Click "Search" on the first line
=> Without this patch you get an ugly screen: https://snag.gy/U8X3ZK.jpg
=> With this patch applied you will not be able to submit without
filling the input in

QA note: Yes, we could handle that controller side as well, but much
more work.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>