Fix an error for cases where the key value is undefined

[budnix]

Context

The PR fixes an error that could happen when the keyboard event was triggered with a key property as undefined. That could happen when the event was emulated by the 3rd party developer's app or IME.

How has this been tested?

I tested the changes locally and I covered the fix with a new test.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Related issue(s):

1. fixes https://github.com/handsontable/dev-handsontable/issues/2096

Affected project(s):

• handsontable

Checklist:

• I have reviewed the guidelines about Contributing to Handsontable and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement

[codesandbox-ci]

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Latest deployment of this branch, based on commit ec5ae27:

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handsontable-ts-demo	Configuration
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[github-actions]

Launch the local version of documentation by running:

npm run docs:review ec5ae27a238299b4baad45d1134d69a1b33ebf77