[paybutton spec] Updates to spec

Summary:

• Specify data pushes will be utf8 encoded
• Specify that all pushes are required. OP_0 may be pushed in the data or nonce position to signify these pushes are empty.

Ref https://github.com/PayButton/paybutton/pull/340

Test Plan: Confirm spec is accurate + desirable

Reviewers: #bitcoin_abc, Klakurka

Reviewed By: #bitcoin_abc, Klakurka

Subscribers: Klakurka

Differential Revision: https://reviews.bitcoinabc.org/D15223