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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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