Fix problem in error handling when no api-doc is available

[taban03]

Signed-off-by: at670475 andrea.tabone@broadcom.com

Description

If the api doc was not retrieved for some reason and was null, a 200 HTTP status code was returned without any clear message. Instead now an API message is returned with 500 return code.
Fixes #571

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

For more details about how should the code look like read the Contributing guideline

[sonarqubecloud]

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