Fix PSBT serialization: remove stray 0x00 when there are no inputs or no outputs (bip174 patch)

[jstortoise]

Summary

PSBTs built with empty input lists and/or empty output lists were serialized with one extra 0x00 byte per empty side. That came from bip174’s keyValsOrEmptyToBuffer, which treated “no maps” as a single zero-length map delimiter. That does not match a strict BIP-174-style layout and can break interoperability with parsers in other stacks that expect the canonical byte length.

What we changed

• Added a patch-package patch on bip174@3.0.0 so that when there are no input maps or no output maps, we append nothing instead of [Uint8Array.from([0])].
• Wired postinstall to patch-package so a clean npm install reapplies the patch.
• Updated test/fixtures/psbt.json for the addInput / “should be equal” case so the expected base64 matches the corrected serialization.

Testing

• Full unit suite: npx mocha 'test/*.ts'.

Follow-up

• When upstream bip174 releases a version with this fix, we should bump the dependency, delete patches/bip174+3.0.0.patch, and remove postinstall / patch-package if no other patches remain.