chore(deps): update dependency null-loader to v4 (#391)

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) |

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### 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>

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packages/google-cloud-texttospeech/package.json

@@ -56,7 +56,7 @@
     "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",