chore(deps): update dependency null-loader to v4 (#244)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [null-loader](https://togithub.com/webpack-contrib/null-loader) | devDependencies | major | [`^3.0.0` -> `^4.0.0`](https://renovatebot.com/diffs/npm/null-loader/3.0.0/4.0.0) |

---

### Release Notes

<details>
<summary>webpack-contrib/null-loader</summary>

### [`v4.0.0`](https://togithub.com/webpack-contrib/null-loader/blob/master/CHANGELOG.md#&#8203;400-httpsgithubcomwebpack-contribnull-loadercomparev300v400-2020-04-15)

[Compare Source](https://togithub.com/webpack-contrib/null-loader/compare/v3.0.0...v4.0.0)

##### Bug Fixes

-   support `webpack@5`

##### ⚠ BREAKING CHANGES

-   minimum required Nodejs version is `10.13`

</details>

---

### Renovate configuration

:date: **Schedule**: "after 9am and before 3pm" (UTC).

:vertical_traffic_light: **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

:recycle: **Rebasing**: Whenever PR is behind base branch, or you tick the rebase/retry checkbox.

:no_bell: **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

---

This PR has been generated by [WhiteSource Renovate](https://renovate.whitesourcesoftware.com). View repository job log [here](https://app.renovatebot.com/dashboard#googleapis/nodejs-scheduler).

Author: renovate-bot

packages/google-cloud-scheduler/package.json

@@ -54,7 +54,7 @@
     "jsdoc-region-tag": "^1.0.2",
     "linkinator": "^2.0.0",
     "mocha": "^7.0.0",
-    "null-loader": "^3.0.0",
+    "null-loader": "^4.0.0",
     "pack-n-play": "^1.0.0-2",
     "sinon": "^9.0.1",
     "ts-loader": "^7.0.0",