fix: Omit Empty Assertion Fields in Device Authorization Request

[jonyoder]

Connected to #744.

This adds the omitempty attribute to the client assertion fields used for device authorization. These fields are encoded as form values when calling the device/authorize API. When these fields are empty and don't include the omitempty attribute, they're being encoded as empty/blank values which can cause issues with some providers, e.g., Okta.

Please let me know if there's anything else I can do to help. I didn't see any relevant unit tests to update.

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