Handle when the value type is not known when serializing shapes

[justinchuby]

When the value type is not known, we don't know where to store the shape inside a type proto. This change will make the serializer skip serializing the shape instead of error out.

Fix #202

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.78%. Comparing base (e6a9b8b) to head (7e9e1f0).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/onnx_ir/serde.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #203      +/-   ##
==========================================
- Coverage   76.78%   76.78%   -0.01%     
==========================================
  Files          40       40              
  Lines        4903     4914      +11     
  Branches      976      978       +2     
==========================================
+ Hits         3765     3773       +8     
- Misses        853      856       +3     
  Partials      285      285              

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