Addon docs: Add safe check in Webpack preset

[yannbf]

Closes #23945

What I did

This PR adds a fix to addon-docs in circumstances where users have custom webpack configs which end up not having module.rules, and therefore the following error is thrown:

module2.rules is not iterable

How to test

1. Run a sandbox for template, e.g. yarn task --task sandbox --start-from auto --template react-webpack/18-ts
2. Add a webpack final that removes module.rules
3. Run Storybook

It should not break

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

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts
• Make sure this PR contains one of the labels below.

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

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

[kasperpeulen]

LGTM

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