You can use the todict() method on the built-in fields variable in a Transform node's ConfigureFields script to construct a dictionary representation of the input fields. You can then check to see whether the required fieldname is present as a key in the dictionary. If it is, the required field is present; if not, the field can be added to the output metadata.
If you don't assign a value to the output field it will have a value of Null, but you can explicitly assign a value of Null to the output if you want to do so for completeness.

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Adrian Williams
Precisely Software Inc.
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