perf(ui): prevent blockType: "$undefined" from being sent through the network (#12131)

Removes `$undefined` strings from being sent through the network when
sending form state requests. When adding new array rows, we assign
`blockType: undefined` which is stringified to `"$undefined"`. This is
unnecessary, as simply not sending this property is equivalent, and this
is only a requirement for blocks. This change will save on request size,
albeit minimal.

| Before | After |
|--|--|
|<img width="1267" alt="Untitled"
src="https://github.com/user-attachments/assets/699f38bd-7db9-4a52-931d-084b8af8530f"
/> | <img width="1285" alt="image"
src="https://github.com/user-attachments/assets/986ecd4c-f22d-4143-ad38-0c5f52439c67"
/> |

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

@@ -28,11 +28,14 @@ export function fieldReducer(state: FormState, action: FieldAction): FormState {
 
       const newRow: Row = {
         id: (subFieldState?.id?.value as string) || new ObjectId().toHexString(),
-        blockType: blockType || undefined,
         collapsed: false,
         isLoading: true,
       }
 
+      if (blockType) {
+        newRow.blockType = blockType
+      }
+
       withNewRow.splice(rowIndex, 0, newRow)
 
       if (blockType) {