[cli][build] Relax the git id plugin

fixes #6072 (closed)

When using git worktree, the .git directory is in a different location than configured in the git id plugin.

This PR relaxes the errors that may be exposed by the git id plugin and protects against NPE when it does not generate version information.

This will cause the long-form version information to display like:

openapi-generator-cli 4.3.1-SNAPSHOT
  commit : unknown
  built  : -999999999-01-01T00:00:00+18:00
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

Which should be fine.

An alternative might be for users to pass -Dmaven.gitcommitid.skip=true, but this would require documentation which users may not easily find. Opting for dropping the supplemental version information seems a better approach from the user perspective.

If contributors start using worktrees regularly, we'll look into another option.

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