check that a separator is found for psbt inputs, outputs, and global map

Summary:
PR description:
Currently it doesn't make sure that a separator was found so PSBTs missing a trailing separator would still pass. This fixes that and adds a test case for it.

It really only makes sense to check for the separator for the output maps as if an input or global map was missing a separator, the fields following it would be interpreted as belonging to the previous input or global map. However I have added the check for those two anyways to be consistent.

Backport of Core PR14377

Test Plan: ninja && test/functional/test_runner.py rpc_psbt

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7753