DownloadImageModal: Add support for empty string OS variant

[thgreasi]

This is in order to match the actual Release.variant field in open-balena.

Change-type: patch See: https://github.com/balena-io/balena-sdk/pull/1232

Connects-to: # See: Depends-on: Change-type: major|minor|patch

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Contributor checklist

• [ ] I have regenerated jest snapshots for any affected components with npm run generate-snapshots

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[thgreasi]

I self-certify!