fix: beforeValidate deleting value when access returns false (#11549)

### What?
Regression caused by https://github.com/payloadcms/payload/pull/11433 

If a beforeChange hook was checking for a missing or undefined `value`
in order to change the value before inserting into the database, data
could be lost.

### Why?
In #11433 the logic for setting the fallback field value was moved above
the logic that cleared the value when access control returned false.

### How?
This change ensures that the fallback value is passed into the
beforeValidate function _and_ still available with the fallback value on
siblingData if access control returns false.

Fixes https://github.com/payloadcms/payload/issues/11543