Pre-conditions:
Required PR: https://github.com/ibexa/dashboard/pull/107

Steps to reproduce:
1. Edit dashboard.
2. Go to News block and click Settings.
3. In the Number of news field, type boundary value 0.
4. Click Save and close.

Expected result:
A validation error appears and prevents from saving changes.
(For example, Recent activity block, see screenshot).

Actual result:
No validation  on 0 value.