[BUG]: Fix null value inserted into mqtt client map

[kaushalaroraharman]

What happened?

During shutdown of application, and closing of the MQTT clients, the mqtt client map is being inserted with a null value against a given platform. Since the map is of type concurrent hash map, which does not allow null values, this is resulting in exception during shutdown of application.

Versions

1.0.0

Relevant log output

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