Bug: Flushing empty metrics prints JSON with dimension values

Hello team,

Philipp here from Powertools for AWS Lambda (Java). I found an issue where EMF logs a JSON object when flushing even if the customer did not add any metrics. I believe that this is a bug but wanted to confirm with you if this is expected.

Related issue in Powertools for AWS Lambda (Java) repository: https://github.com/aws-powertools/powertools-lambda-java/issues/1890

For example, if I do this:

```java
emfLogger = new software.amazon.cloudwatchlogs.emf.logger.MetricsLogger(environmentProvider,
metricsContext);

emfLogger.setNamespace("MyNamespace");
emfLogger.putDimensions(DimensionSet.of("Service", "MyService");
emfLogger.flush()
```

I see this in my logs when running on AWS Lambda with Java 11 runtime:

```json
{"Service":"MyService"}
```

My expectation is that nothing should be logged because I did not call `putMetric` at any point. What do you think?