Steps to reproduce:

1. Go to host
2. Open Value mapping tab.
3. Add value mapping with: type - regexp, value - empty field, mapping - add something.
4. Press update in value mapping overlay.
5. Press update in host.

Result:
Value mapping with empty regex value field, can be added in value mapping tab.

But, pressing Update button in host, error appears.

Expected:
Erro should appear after pressing Update button in value mappings.