Update dependency @ember/test-helpers to v2.9.4

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
@ember/test-helpers	2.9.3 -> 2.9.4

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Release Notes

emberjs/ember-test-helpers (@​ember/test-helpers)

### [`v2.9.4`](https://togithub.com/emberjs/ember-test-helpers/releases/tag/v2.9.4): Release 2.9.4 [Compare Source](https://togithub.com/emberjs/ember-test-helpers/compare/v2.9.3...v2.9.4) ##### :bug: Bug Fix - [#​1418](https://togithub.com/emberjs/ember-test-helpers/pull/1418) (backport) bugfix: handle `null` and `undefined` in `setProperties` ([@​chriskrycho](https://togithub.com/chriskrycho)) ##### Committers: 1 - Chris Krycho ([@​chriskrycho](https://togithub.com/chriskrycho))

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