fix: Panic on root $id without base

[Stranger6667]

Fixes #547

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.38%. Comparing base (d07e47f) to head (3b8160c).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
crates/jsonschema/src/retriever.rs	42.85%	4 Missing ⚠️
crates/jsonschema-referencing/src/registry.rs	93.75%	1 Missing ⚠️
crates/jsonschema/src/output.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
+ Coverage   86.37%   86.38%   +0.01%     
==========================================
  Files          88       88              
  Lines       12392    12395       +3     
==========================================
+ Hits        10703    10708       +5     
+ Misses       1689     1687       -2     

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[codspeed-hq]

CodSpeed Performance Report

Merging #556 will degrade performances by 10.57%

_{Comparing dd/fix-ref-panic (3b8160c) with master (d07e47f)}

Summary

❌ 1 regressions
✅ 38 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	dd/fix-ref-panic	Change
❌	is_valid[Fast/Invalid]	740 ns	827.5 ns	-10.57%