fix(ui): prevent error if rows is undefined in mergeServerFormState (#12962)

### What? 
Adds optional chaining when accessing `rows` in `mergeServerFormState`
to prevent error crashing the UI.

### Why? 
When an array field is populated in a `beforeChange` hook and was
previously empty, it crashes `mergeServerFormState.ts` on this line
because no `rows` exist:

```ts 
const indexInCurrentState = currentState[path].rows.findIndex
``` 

The line after this checks `if (indexInCurrentState > -1)` so returning
undefined here will not affect the subsequent code.

### How? 
Added optional chaining to the access of `rows`, which prevents the
error being thrown.

Fixes #12944

Author: jessrynkar

packages/ui/src/forms/Form/mergeServerFormState.ts

@@ -50,7 +50,7 @@ export const mergeServerFormState = ({
       newState[path].rows = [...(currentState[path]?.rows || [])] // shallow copy to avoid mutating the original array
 
       incomingField.rows.forEach((row) => {
-        const indexInCurrentState = currentState[path].rows.findIndex(
+        const indexInCurrentState = currentState[path].rows?.findIndex(
           (existingRow) => existingRow.id === row.id,
         )