Bug 16550: Can't set opac news expiration date to NULL, it reverts to today

Bug 16550: Can't set opac news expiration date to NULL, it reverts to today

This patch fixes an issue with the expiration dates for news always reverting to today if empty.

To test:
- Apply patch
- Go to Home > Tools > News
- Create a news item, do not set expiration date
- Verify that expiration date stays empty
- Edit this news item
- Do not set expiration date
- Verify that expiration date stays empty
- Do the same with expiration dates
- Verify that they are saved properly

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>