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]

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 88.84%. Comparing base (5314ebb) to head (35a1693). :warning: 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 :warning:

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%)	:arrow_down:

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