Merge #11900: [script] simplify CheckMinimalPush checks, add safety assert

Summary:
0749808a7 CheckMinimalPush comments are prescriptive (Gregory Sanders)
176db6147 simplify CheckMinimalPush checks, add safety assert (Gregory Sanders)

Pull request description:

the two conditions could simply never be hit as `true`, as those opcodes have a push payload of size 0 in `data`.

Added the assert for clarity for future readers(matching the gating in the interpreter) and safety for future use.

This effects policy only.

Tree-SHA512: f49028a1d5e907ef697b9bf5104c81ba8f6a331dbe5d60d8d8515ac17d2d6bfdc9dcc856a7e3dbd54814871b7d0695584d28da6553e2d9d7715430223f0b3690

Backport of Core PR11900
https://github.com/bitcoin/bitcoin/pull/11900/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc, markblundeberg

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3761