Properly pass the AbstractField internal value to shared state in Slider and ProgressIndicator (#10921)

* In case of a null value, 0 is stored to shared state. This is consistent with how CheckBox is already implemented.
* This somewhat hacky solution should be replaced with a better one once AbstractField itself is migrated. See #11064.

Change-Id: I2b313af8491a6deccdc7a509dcd1b718482cdcd4