fix: properly initialize rabbit health check

[aneojgurhem]

Motivation

Properly initialize rabbit so that its health check does not return Unhealthy all the time preventing the container from starting when there is a health check set up.

Description

Variable isInitialized_ was not set to true, forcing the health check to be false.

This variable was not needed as it is enough to check if the model (represents the underlying connection) is not null. When the model is null, the plugin is not initialized yet. This pr replace the variable by the proper checks on the model.

Testing

Health checks added by #785 were used to validate that this development properly initialises the health check.

Impact

RabbitMQ plugin initialization now works, allowing the usage of the plugin.

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• I have thoroughly tested my modifications and added tests when necessary.
• Tests pass locally and in the CI.
• I have assessed the performance impact of my modifications.