Null banner JSON message fix

[jp-tosca]

Which issue(s) this PR closes:

Closes #10638

Suggestions on how to test this:

• curl -H "Content-type:application/json" -X POST "http://localhost:8080/api/admin/bannerMessage" --data "{}"
• This can also be tested with postman by calling http://localhost:8080/api/admin/bannerMessage POST with no body or attached file.

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

Is there a release notes update needed for this change?:
No, it is an improvement of an error message since the current one display Java/JSON objects as part of the message

[coveralls]

coverage: 20.657% (-0.004%) from 20.661%
when pulling 1867e30 on banner_null_json_message_fix
into 853965e on develop.

[github-actions]

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