psbt: allow empty bip32 derivation path

[guggero]

Fixes lightningnetwork/lnd#6372.

BIP-0174 defines the derivation path being encoded as
"<32-bit uint> <32-bit uint>*"
with the asterisk meaning 0 to n times. Which in turn means that an
empty path is valid, only the key fingerprint is mandatory.

cc @Roasbeef @ellemouton

[coveralls]

Pull Request Test Coverage Report for Build 2051021599

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.003%) to 54.19%

---

Files with Coverage Reduction	New Missed Lines	%
connmgr/connmanager.go	2	86.79%

Totals
Change from base Build 2006933401:	0.003%
Covered Lines:	24769
Relevant Lines:	45708

---

💛 - Coveralls

[Roasbeef]

LGTM 🎄

We'll need to do a btcutil tag after this is in, so it's visible to external consumers using the tags.

[Roasbeef]

👍

[Roasbeef]

I pushed the btcutil/psbt/v1.1.2 tag.

[coveralls]

Pull Request Test Coverage Report for Build 2051021599

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.003%) to 54.19%

---

Files with Coverage Reduction	New Missed Lines	%
connmgr/connmanager.go	2	86.79%

Totals
Change from base Build 2006933401:	0.003%
Covered Lines:	24769
Relevant Lines:	45708

---

💛 - Coveralls