Allow pubspec.lock be missing

[fzyzcjy]

Changes

Close #2281

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• If this PR adds/changes features, documentations (in the ./website folder) are updated.
• CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov]

Codecov Report

Attention: Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.97%. Comparing base (9ab7435) to head (afb76c3).
Report is 20 commits behind head on master.

Files with missing lines	Patch %	Lines
...gen/src/library/utils/dart_repository/dart_repo.rs	96.96%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2289      +/-   ##
==========================================
- Coverage   98.97%   98.97%   -0.01%     
==========================================
  Files         490      490              
  Lines       20276    20281       +5     
==========================================
+ Hits        20069    20073       +4     
- Misses        207      208       +1     

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