test: add test for offer parsing with empty paths and valid issuer

[erickcestari]

Verifies that offers with empty paths are accepted as long as the issuer_id is set.

Follow-up: #4018

[ldk-reviews-bot]

👋 I see @joostjager was un-assigned.
If you'd like another reviewer assignment, please click here.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.84%. Comparing base (5314ebb) to head (35a1693).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/offers/offer.rs	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4020      +/-   ##
==========================================
- Coverage   88.86%   88.84%   -0.02%     
==========================================
  Files         175      175              
  Lines      127725   127731       +6     
  Branches   127725   127731       +6     
==========================================
- Hits       113501   113486      -15     
- Misses      11662    11684      +22     
+ Partials     2562     2561       -1     

Flag	Coverage Δ
fuzzing	21.76% <ø> (ø)
tests	88.68% <66.66%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[TheBlueMatt]

nit: just use builder.clear_paths()? If you really want you could even check that the offer built in pays_for_offer_without_blinded_paths round-trips.

[erickcestari]

The ideia is to test this specifc scenario:

Where path is Some(empty_vector) and with a valid issuer_id

since we moved from:

let (issuer_signing_pubkey, paths) = match (issuer_id, paths) {
			(None, None) => return Err(Bolt12SemanticError::MissingIssuerSigningPubkey),
			(_, Some(paths)) if paths.is_empty() => return Err(Bolt12SemanticError::MissingPaths),
			(issuer_id, paths) => (issuer_id, paths),
		};

to:

let (issuer_signing_pubkey, paths) = match (issuer_id, paths) {
			(None, None) => return Err(Bolt12SemanticError::MissingIssuerSigningPubkey),
			(None, Some(paths)) if paths.is_empty() => {
				return Err(Bolt12SemanticError::MissingPaths)
			},
			(issuer_id, paths) => (issuer_id, paths),
		};

so before the update this new test would fail.

builder.clear_paths() would insert None into offer.paths which isn't the case we want to test

	#[cfg_attr(c_bindings, allow(dead_code))]
	pub(crate) fn clear_paths($($self_mut)* $self: $self_type) -> $return_type {
		$self.offer.paths = None;
		$return_value
	}

[TheBlueMatt]

Mm, then we need to test actually paying such a blinded path, not just decoding it.

[ldk-reviews-bot]

👋 The first review has been submitted!

Do you think this PR is ready for a second reviewer? If so, click here to assign a second reviewer.