[ecash bip21] Make empty OP_RETURN out of spec

Summary:
I can't think of a good reason to include an empty OP_RETURN output. I could though imagine it happening by accident if someone is generating bip21 params.

Make this out of spec so implementer validation can catch it.

Test Plan: Review summary, attempt to think of counterexample good reason for empty opreturn in params

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15059