fix: crash with non svelte files in `svelte/no-unused-class-name`

[ota-meshi]

fixes #512

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Latest commit: a7e572af45bcb9c1525e855d0aedcfff4d5ed721

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

This PR includes changesets to release 1 package

| Name | Type | | -------------------- | ----- | | eslint-plugin-svelte | Patch |

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