Fix bugs w.r.t. test-engine

[juileetikekar]

Description

This PR consists of fixes for the bugs found during API validation with aas-test-engine.

Motivation and Context

This change is required for the correct implementation of the APIs

Fixes # (issue)

1. In case of POST APIs, if the request body has serialization issues, the server now sends BadRequest (earlier Internal Server Error).
2. In case of API GET "/shells/$reference", if there are no shells present, server used to throw InternalServerError, due to null pointer exception. The issue has been now fixed. The server in this case now return empty Result.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

The fixes are tested manually with the help of in-buit swagger-ui and with aas-test-engine.

Screenshots (if appropriate):

Checklist:

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works