fix(hmr): __HMR_PORT__ should not be 'undefined'

[sapphi-red]

Description

When config.server.hmr exsits and both config.server.hmr.clientPort and config.server.hmr.port does not exist, __HMR_PORT__ was injected as 'undefined' instead of null.

For example a config like this:

export default {
  server: {
    hmr: {}
  }
}

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

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