Source-loader: Fix property key validation

[MrZillaGold]

What I did

Checking if the property has a key to export story-file parameters as a constant.

export const story: Meta<...> = {};

export default story;

Preview build error:

This file has the same check for a similar place:

https://github.com/storybookjs/storybook/blob/main/code/lib/source-loader/src/abstract-syntax-tree/traverse-helpers.js#L54

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
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• Add or update documentation reflecting your changes
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  MIGRATION.MD

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  • bug: Internal changes that fixes incorrect behavior.
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This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

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[yannbf]

Hey @MrZillaGold thanks for your contribution! Great catch!