CLI: only add dependencies on sb init if they do not exist

[yannbf]

Issue: N/A

What I did

When running sb init -f and no dependencies are required to be installed (because they already exist in package.json), it would crash in pnpm:

This PR fixes that, making sure that the "add dependencies" step only runs if needed

How to test

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]