Netty HttpClient changes

[alzimmermsft]

Description

Two minor Netty HttpClient changes:

• Only override AddressResolverGroup if our custom HTTP proxy handler was added.
• Check if the Netty Channel is registered, in an EventLoop, before attempting to use the EventLoop.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[azure-sdk]

API change check

API changes are not detected in this pull request.

[alzimmermsft]

/check-enforcer override