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

test/access-control/collections/hooks/index.ts

@@ -0,0 +1,44 @@
+import type { CollectionConfig } from 'payload'
+
+import { hooksSlug } from '../../shared.js'
+
+export const Hooks: CollectionConfig = {
+  slug: hooksSlug,
+  access: {
+    update: () => true,
+  },
+  fields: [
+    {
+      name: 'cannotMutateRequired',
+      type: 'text',
+      access: {
+        update: () => false,
+      },
+      required: true,
+    },
+    {
+      name: 'cannotMutateNotRequired',
+      type: 'text',
+      access: {
+        update: () => false,
+      },
+      hooks: {
+        beforeChange: [
+          ({ value }) => {
+            if (!value) {
+              return 'no value found'
+            }
+            return value
+          },
+        ],
+      },
+    },
+    {
+      name: 'canMutate',
+      type: 'text',
+      access: {
+        update: () => true,
+      },
+    },
+  ],
+}