Refactor BaseConnectionPool empty method

[bandinib-amzn]

Description

ConnectionPool.empty() never calls callback when there is no connection. Refactor empty method so that callback will get always called.

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

Check List

• [X] New functionality includes testing.
  • [X] All tests pass
• [X] Linter check was successfull - yarn run lint doesn't show any errors
• [X] Commits are signed per the DCO using --signoff
• [X] Changelog was updated.

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[dblock]

Looks good, needs a test please.

[bandinib-amzn]

Looks good, needs a test please.

Added UT with no connection.