fix: previousValue from hooks should be populated within lexical blocks (#14856)

### What?
Updates the `previousValue` data sent to the `afterChange` hook to
accomodate **lexical nested** paths.

### Why?
Currently the `afterChange` hook uses the previousDoc:
```ts
previousValue: getNestedValue(previousDoc, pathSegments) ?? previousDoc?.[field.name],
```

However, with fields from within Lexical, the data on the full
`previousDoc` has a complex nesting structure. In this case, we should
be using the `previousSiblingDoc` to only return the sibling data and
easily de-structure the data.

### How?
Checks if sibling data exists, and uses it if it does:
```ts
 const previousValData =
    previousSiblingDoc && Object.keys(previousSiblingDoc).length > 0
      ? previousSiblingDoc
      : previousDoc
```

**Reported by client.**

Related PR: https://github.com/payloadcms/payload/pull/14582

---------

Co-authored-by: German Jablonski <43938777+GermanJablo@users.noreply.github.com>