3.35DataWeave isn't validation

This distinction will make you sound mature.

You can write things like:

if (payload.customerId == null)
    ...

But if customerId is required, ideally validation happens as a defined input-validation/API-contract concern.

Think:

VALIDATION

Is this request acceptable?
        ↓

TRANSFORMATION

How do I map an acceptable request?

Don't turn a DataWeave transformation into a giant pile of validation logic.