error in query with multiple resolvers at root nulls out all data

**schema** (with Amplify annotation shorthand)
```
type A { id }
type B { id }
query {
get_A:A @function(...)
get_B:B @function(...)
}
```

**client request**
```
query {
get_A { id }
get_B { id }
}
```

## Expected Behavior
- get_A and get_B run in parallel
- if an error is in one resolver, the data set would be `null` and the error would be found in the `errors` struct
```
{
"data": {
"get_A":
{
"id": "1"
},
"get_B": null,
"errors": ...
```

## Actual Behavior
Data is `null` even though `get_A` resolver completed successfully.

```
{
"data": null,
"errors": [
{
"path": [
"get_B"
]
...
}
```

Reproduce the issue using the schema and client request shown in the report, then compare the successful get_A resolver with the failing get_B response. Done means independent root resolvers retain successful data while the failed resolver appears as null and its error remains in the errors response; no source files or tests are identified in the issue.

Written by the indexing model from the issue text.