Allow null data in place of steps

[tonyhb]

Summary

Allows steps to return null data, eg. within the executor to denote things as complete.

• [x] Added a docs PR that references this PR
• [x] Added unit/integration tests
• [x] Added changesets if applicable

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 028b483a9a5810b3941a1913d0f8c7a8744847af

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

This PR includes changesets to release 1 package

| Name | Type | | ------- | ----- | | inngest | Patch |

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