fix NPE in ScriptedOutgoingMapping when value from script resolved to null

[thjaeckle]

Encountered through failing SystemTests on master branch:
https://github.com/eclipse-ditto/ditto/runs/35877513588#r25

[thjaeckle]

Down from 6 Test failures to 4:
https://github.com/eclipse-ditto/ditto/runs/35883171669#r32

Will investigate further why the 4 remain ..

[thjaeckle]

It seems like message headers for AMQP 1.0 and RabbitMQ are somehow lowercased.

Before the integration test ensureSourceAuthenticationPlaceholderSubstitution() did send a header named headerId - and used that header in a placeholder substitution.
Running the test locally I found out that this header is received at Ditto as headerid so the capital I gets lowercases to i.
I found the same for messageId header - lowercased to messageid.

For MQTT5 e.g. this test passes, so case is preserved there.

@alstanchev do you have any idea why this could happen?
Maybe a dependency update?

[alstanchev]

That is strange.
No idea, doesn't ring any bells.

[thjaeckle]

@alstanchev I found it .. a DittoHeadersBuilder lowercases by default all read external header keys.
And I used DittoHeadersBuilder to remove "traceparent" header in order to fix the trace span-hierarchy.

Fixed in another commit to this PR.

If you could have a look, that would be great.
I will also run system tests again.

[thjaeckle]

System tests are green:
https://github.com/eclipse-ditto/ditto/actions/runs/12882019337

[alstanchev]

LGTM 👍