🐛 Source Amazon Seller Partner: attempt to fix an issue with role_arn required with authorization is done via oauth

[andreyAtBB]

What

When user creates Amazon Seller Partner connector source on Airbyte Cloud, authenticate via Oauth and save and error appears as described in these issues:
#28727
#27495

How

If role_arn is not provided (which is the case for Oauth authentication) do not attempt to split it and simply proceed.

Recommended reading order

n/a

🚨 User Impact 🚨

Should not have any impact but should allow user to proceed one step further during connector set up.

Pre-merge Actions

None

[CLAassistant]

All committers have signed the CLA.

[github-actions]

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[rileybrook]

@lazebnyi Hoping this small PR can be shepherded through as it blocks an Airbtye Cloud customer from onboarding new clients.

[andreyAtBB]

@marcosmarxm, there are some workflows awaiting for approval from a maintainer blocking the ability to merge the fix in.
Is this something you can look at?

[marcosmarxm]

@andreyAtBB I'm running tests right now to release the change.