…cript strict mode is on

When importing `@apollo/client` in a project with TS 4.9, graphql 16.6 and `strict: true`, we see the following compilation error:

```
./node_modules/@apollo/client/link/core/types.d.ts(44,18): error TS2430: Interface 'SingleExecutionResult<TData, TContext, TExtensions>' incorrectly extends interface 'ExecutionResult<TData, TExtensions>'.
  Types of property 'data' are incompatible.
    Type 'Data<TData>' is not assignable to type 'TData | null'.
      Type 'undefined' is not assignable to type 'TData | null'.
```

Because `Data<TData>` expands to:
```ts
  data?: TData | undefined | null
```

Which is subtly different from:
```ts
  data?: TData | null
```

This fixes the type to be compatible.