Update codegen default behavior for is-modular-library true

[v-jiaodi]

fixes #3350

[microsoft-github-policy-service]

Hi @@v-jiaodi. Thank you for your interest in helping to improve the Azure SDK experience and for your contribution. We've noticed that there hasn't been recent engagement on this pull request. If this is still an active work stream, please let us know by pushing some changes or leaving a comment. Otherwise, we'll close this out in 7 days.

[microsoft-github-policy-service]

Hi @@v-jiaodi. Thank you for your contribution. Since there hasn't been recent engagement, we're going to close this out. Feel free to respond with a comment containing "/reopen" if you'd like to continue working on these changes. Please be sure to use the command to reopen or remove the "no-recent-activity" label; otherwise, this is likely to be closed again with the next cleanup pass.