observeQuery errors without including selectionSet

### Before opening, please confirm:

- [x] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-js/issues?q=is%3Aissue+) and [discussions](https://github.com/aws-amplify/amplify-js/discussions).
- [x] I have read the guide for [submitting bug reports](https://github.com/aws-amplify/amplify-js/blob/main/CONTRIBUTING.md#bug-reports).
- [x] I have done my best to include a minimal, self-contained set of instructions for consistently reproducing the issue.

### JavaScript Framework

React

### Amplify APIs

GraphQL API

### Amplify Version

v6

### Amplify Categories

api

### Backend

Amplify Gen 2

### Environment information

```
# Put output below this line
System:
OS: macOS 15.5
CPU: (8) arm64 Apple M1 Pro
Memory: 106.42 MB / 16.00 GB
Shell: 5.9 - /bin/zsh
Binaries:
Node: 22.12.0 - ~/.nvm/versions/node/v22.12.0/bin/node
Yarn: 1.22.19 - /usr/local/bin/yarn
npm: 11.6.1 - ~/.nvm/versions/node/v22.12.0/bin/npm
Watchman: 2025.05.26.00 - /opt/homebrew/bin/watchman
Browsers:
Brave Browser: 123.1.64.116
Chrome: 140.0.7339.215
Edge: 140.0.3485.94
Safari: 18.5
npmPackages:
@aws-amplify/backend: ^1.16.1 => 1.16.1
@aws-amplify/backend-cli: ^1.8.0 => 1.8.0
@aws-amplify/ui-react: ^6.12.0 => 6.12.0
@aws-amplify/ui-react-internal: undefined ()
@aws-amplify/ui-react-server: undefined ()
@aws-sdk/client-eventbridge: ^3.891.0 => 3.891.0
@aws-sdk/lib-dynamodb: ^3.888.0 => 3.890.0
@aws-sdk/util-dynamodb: ^3.888.0 => 3.890.0
@dotenvx/dotenvx: ^1.49.1 => 1.49.1
@headlessui/react: ^2.2.8 => 2.2.8
@heroicons/react: ^2.2.0 => 2.2.0
@reduxjs/toolkit: ^2.9.0 => 2.9.0
@reduxjs/toolkit-query: 1.0.0
@reduxjs/toolkit-query-react: 1.0.0
@reduxjs/toolkit-react: 1.0.0
@tailwindcss/forms: ^0.5.10 => 0.5.10
@tailwindcss/postcss: ^4.1.13 => 4.1.13
@tailwindcss/vite: ^4.1.13 => 4.1.13
@testing-library/jest-dom: ^6.8.0 => 6.8.0
@testing-library/react: ^16.3.0 => 16.3.0
@types/aws-lambda: ^8.10.152 => 8.10.152
@types/crypto-js: ^4.2.2 => 4.2.2
@types/react: ^19.1.10 => 19.1.13
@types/react-dom: ^19.1.7 => 19.1.9
@types/react-modal: ^3.16.3 => 3.16.3
@types/redux-mock-store: ^1.5.0 => 1.5.0
@types/testing-library__jest-dom: ^5.14.9 => 5.14.9
@types/testing-library__react: ^10.0.1 => 10.0.1
@typescript-eslint/eslint-plugin: ^6.2.1 => 6.21.0
@typescript-eslint/parser: ^6.2.1 => 6.21.0
@vis.gl/react-google-maps: ^1.5.5 => 1.5.5
@vitejs/plugin-react: ^5.0.2 => 5.0.3
@vitejs/plugin-react-swc: ^4.0.0 => 4.1.0
autoprefixer: ^10.4.21 => 10.4.21
aws-amplify: ^6.15.6 => 6.15.6
aws-amplify/adapter-core: undefined ()
aws-amplify/adapter-core/internals: undefined ()
aws-amplify/analytics: undefined ()
aws-amplify/analytics/kinesis: undefined ()
aws-amplify/analytics/kinesis-firehose: undefined ()
aws-amplify/analytics/personalize: undefined ()
aws-amplify/analytics/pinpoint: undefined ()
aws-amplify/api: undefined ()
aws-amplify/api/internals: undefined ()
aws-amplify/api/server: undefined ()
aws-amplify/auth: undefined ()
aws-amplify/auth/cognito: undefined ()
aws-amplify/auth/cognito/server: undefined ()
aws-amplify/auth/enable-oauth-listener: undefined ()
aws-amplify/auth/server: undefined ()
aws-amplify/data: undefined ()
aws-amplify/data/server: undefined ()
aws-amplify/datastore: undefined ()
aws-amplify/in-app-messaging: undefined ()
aws-amplify/in-app-messaging/pinpoint: undefined ()
aws-amplify/push-notifications: undefined ()
aws-amplify/push-notifications/pinpoint: undefined ()
aws-amplify/storage: undefined ()
aws-amplify/storage/s3: undefined ()
aws-amplify/storage/s3/server: undefined ()
aws-amplify/storage/server: undefined ()
aws-amplify/utils: undefined ()
aws-cdk-lib: ^2.215.0 => 2.215.0
constructs: ^10.4.2 => 10.4.2
crypto-js: ^4.2.0 => 4.2.0
date-fns: ^4.1.0 => 4.1.0
esbuild: ^0.25.9 => 0.25.10
eslint: ^8.46.0 => 8.57.1
eslint-config-prettier: ^8.10.0 => 8.10.2
eslint-plugin-prettier: ^5.0.0 => 5.5.4
eslint-plugin-react: ^7.33.1 => 7.37.5
eslint-plugin-react-hooks: ^4.6.0 => 4.6.2
eslint-plugin-tailwindcss: ^3.13.0 => 3.18.2
form-data: ^4.0.4 => 4.0.4
glob: ^11.0.3 => 11.0.3 (7.2.3, 10.4.5)
globals: ^16.3.0 => 16.4.0 (13.24.0)
heroicons: ^2.2.0 => 2.2.0
postcss: ^8.5.6 => 8.5.6
prettier: ^3.6.2 => 3.6.2 (2.3.2, 1.19.1)
react: ^19.1.1 => 19.1.1 (18.3.1)
react-datepicker: ^8.7.0 => 8.7.0
react-dom: ^19.1.1 => 19.1.1 (18.3.1)
react-modal: ^3.16.3 => 3.16.3
react-number-format: ^5.4.4 => 5.4.4
react-redux: ^9.2.0 => 9.2.0
react-router-dom: ^7.9.1 => 7.9.1
react-tooltip: ^5.29.1 => 5.29.1
rimraf: ^6.0.1 => 6.0.1 (3.0.2)
tailwindcss: ^3.4.17 => 3.4.17 (4.1.13)
ts-jest: ^29.4.1 => 29.4.2
tsx: ^4.20.5 => 4.20.5 (4.19.4)
typescript: ^5.9.2 => 5.9.2 (4.4.4, 4.9.5)
vite: ^7.1.2 => 7.1.5
vitest: ^3.2.4 => 3.2.4
npmGlobalPackages:
corepack: 0.29.4
npm: 11.6.1

```

### Describe the bug

I'm following this [observeQuery example in the gen2 docs](https://docs.amplify.aws/javascript/build-a-backend/data/subscribe-data/) and kept getting this error:

`Uncaught TypeError: needle is null` from amplify internal file findIndexByFields.ts

I resolved this by including selectionSet param, listing the properties requested.

### Expected behavior

I was unaware of this selectionSet param because it wasn't in the docs page, and just assumed all properties would be returned.

### Reproduction steps

1. Create subscription observeQuery without the selectionSet specified,
2. do some graphql create or update on the observed table

### Code Snippet

```javascript
// Put your code below this line.
// working example with selectionSet
const querySub = client.models.VendorResult.observeQuery({
filter: { searchSessionID: { eq: searchSessionID } },
selectionSet: ["id", "status", "createdAt"],
authMode: "userPool",
}).subscribe({
next: ({ items, isSynced }) => {
console.log("Subscription received items:", items, "isSynced:", isSynced);

},
error: (err) => console.error("subscription error", err),
});
```

### Log output

```
// Put your logs below this line

Subscription received items:
Array []
isSynced: false [SubscribedVendorList.tsx:57:17](http://localhost:5173/src/components/VendorIndex/FindVendors/SubscribedVendorList.tsx)
Subscription received items:
Array []
isSynced: true [SubscribedVendorList.tsx:57:17](http://localhost:5173/src/components/VendorIndex/FindVendors/SubscribedVendorList.tsx)
Uncaught TypeError: can't access property "id", needle is null
searchObject findIndexByFields.ts:12
findIndexByFields findIndexByFields.ts:12
ingestMessages observeQuery.ts:47
receiveMessages observeQuery.ts:100
next observeQuery.ts:22
RxJS 8
_handleSubscriptionData index.ts:67
_handleIncomingSubscriptionMessage index.ts:427
_registerWebsocketHandlers index.ts:676
onmessage index.ts:641
_initiateHandshake index.ts:631
_initiateHandshake index.ts:617
_establishConnection index.ts:35
retry retry.ts:39
retry retry.ts:16
jitteredExponentialRetry jitteredExponentialRetry.ts:10
_establishRetryableConnection index.ts:592
_initializeWebSocketConnection index.ts:566
_initializeWebSocketConnection index.ts:541
_connectWebSocket index.ts:149
_startSubscriptionWithAWSAppSyncRealTime index.ts:281
startSubscription index.ts:106
subscribe index.ts:127
RxJS 14
observeQuery observeQuery.ts:20
RxJS 4
SubscribedVendorList SubscribedVendorList.tsx:55
React 8
```

### aws-exports.js

_No response_

### Manual configuration

_No response_

### Additional configuration

_No response_

### Mobile Device

_No response_

### Mobile Operating System

_No response_

### Mobile Browser

_No response_

### Mobile Browser Version

_No response_

### Additional information and screenshots

_No response_

Start with findIndexByFields.ts and observeQuery.ts, following the reported stack from observeQuery ingestion into findIndexByFields. Reproduce observeQuery without selectionSet, then compare it with the supplied working example; done means the no-selectionSet case no longer throws and the documented behavior is accurate.

Written by the indexing model from the issue text.