Fix clients not being removed from map during shutdown of application

[kaushalaroraharman]

Resolves #28

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Describe behaviour before the change

• Exception when attempting to add null value in a ConcurrentHashMap

Describe behaviour after the change

• Entry is removed from map, instead of adding null value during clean of mqtt client map

Pull request checklist

• I have read the CONTRIBUTING.md
• My code follows the code style of this project
• Tests for the changes have been added (for bug fixes / features)
• All new and existing tests passed.
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

• Yes
• No

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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