Bug: EMF Metrics Logger emits root dict without metrics when no metrics are added

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Is your feature request related to a problem? Please describe.
When using @FlushMetrics, the default behavior is to log a warning if no metrics emitted. This works correctly. However, when no metrics were added, in addition to logging a warning, the root-level section of the _aws EMF dict is printed. Example:

This is the output when placing @FlushMetrics on a Lambda handler that is not interacting with metrics at all.

{"Service":"payment","logStreamId":"$LATEST","functionVersion":"$LATEST","executionEnvironment":"AWS_Lambda_java11"}

Describe the solution you'd like
If no metrics are added, we should not attempt to flush since this seems to be a bug in the underlying aws-embedded-metrics library.

The expected output is that nothing except for the warning is logged when the user is not emitting any metrics.

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Created PR: #1891

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Created issue in EMF repository: awslabs/aws-embedded-metrics-java#165

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Resolving here since the PR addresses the issue.