omit rule-less variants from classMap

[PeterW570]

This PR prevents styleVariants from generating class names for 'empty' variants. As discussed in #1580 this can be helpful for using as conditions for compoundVariants.

Not sure if you'd classify it a bug, or if it's a change you want to make; but it addresses the issue I found, so have made this PR for your consideration.

[changeset-bot]

🦋 Changeset detected

Latest commit: 56d098c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 11 packages

Name	Type
@vanilla-extract/css	Minor
@vanilla-extract/recipes	Minor
@vanilla-extract/compiler	Patch
@vanilla-extract/integration	Patch
@vanilla-extract/rollup-plugin	Patch
@vanilla-extract/vite-plugin	Patch
@vanilla-extract/esbuild-plugin	Patch
@vanilla-extract/jest-transform	Patch
@vanilla-extract/parcel-transformer	Patch
@vanilla-extract/webpack-plugin	Patch
@vanilla-extract/next-plugin	Patch

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