Why does receiving a null node break the execution of the mutation in @appendNode?

Why does receiving a null node break the execution of the mutation? Would it be possible to just not update this node and close it but complete the mutation?

When I receive a null node in return of my mutation, I receive the following error: "MutationHandlers: Expected target node to exist."

https://github.com/facebook/relay/blob/ee03182fa05430690f3b33dfff5d3426e49fd8dc/packages/relay-runtime/handlers/connection/MutationHandlers.js#L195

```
mutation StockCreateMutation(
$input: StockCreateInput!
$connections: [ID!]!
) {
StockCreate(input: $input) {
stock @appendNode(connections: $connections, edgeTypeName: "StockEdge") {
id
type
quantity
createdAt
createdBy {
id
name
}
product {
id
name
stock
}
}
error
success
}
}
```

Start in packages/relay-runtime/handlers/connection/MutationHandlers.js around line 195 and trace the @appendNode handling for a null returned node. Reproduce the StockCreateMutation with stock returning null, then verify the mutation completes without the “Expected target node to exist” error while leaving the connection unchanged.

Written by the indexing model from the issue text.