fix: handle null schemaType in assignType for composed schemas

[itsmib]

Problem

SwaggerSchemaMapper.assignType calls Set.of(schemaType) when the
swagger Schema has a null type. Set.of rejects null, causing an NPE
for any property whose schema is a composed schema (allOf/oneOf/anyOf)
which legitimately carries no type.

This silently drops channels/operations from the published AsyncAPI
document, the only signal being a WARN log with null as the message.

Changes

• Added null check in assignType before calling Set.of(schemaType)
• Improved warning log in DefaultChannelsService and
  DefaultOperationsService to show exception class name when
  getMessage() returns null

Test

Added mapNullTypeForComposedSchema test in SwaggerSchemaMapperTest
to verify no NPE is thrown for schemas with null type.

Fixes #1834

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[timonback]

Hi @itsmib,
thank you for the fix and the included test to ensure that it is covered long term.

I am not sure about the benefit of logging the name of the exception, in case the message of the exception is null. The exception including stacktrace is logged anyway and part of the log.
The original issue is the missing knowledge, which method/payload triggered the issue. Something that is partly covered by the DEBUG level logging, but can be improved upon.

For this PR, can you revert the logging changes?
The fix and test look great!

[itsmib]

Hey @timonback, Thanks for the review. Logging changes are reverted now.