CAMEL-23497: Fix NPE in ManagedTransformerRegistry.listTransformers()

[gnodet]

CAMEL-23497

Handle null from/to DataType in ManagedTransformerRegistry.listTransformers(). The Transformer.getFrom() and getTo() methods are @Nullable — when a transformer is registered by name only (without explicit from/to types), these can be null, causing an NPE when building the JMX CompositeData.

• Add null checks for from/to in listTransformers(), using empty string when null
• Fix incorrect test assertions that expected "camel:any" for a named transformer with null from/to
• Add dedicated test testListTransformersWithNullFromTo that registers a transformer with null from/to directly in the registry and verifies listTransformers() handles it correctly

[github-actions]

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[apupier]

would be nice to add a test covering this case

EDIT: in fact, this was caught by a test based on https://issues.apache.org/jira/browse/CAMEL-23497

[gnodet]

Claude Code on behalf of Guillaume Nodet

Regarding the "camel:any" → "" change in the existing test assertions:

MyTransformer extends Transformer with no @DataTypeTransformer annotation and is registered via transformer().name("custom").withJava(MyTransformer.class). The Transformer constructor only sets from/to if the annotation is present — so they stay null.

The CAMEL-18698 commit changed the test expectation from assertEquals(null, from) to assertEquals("camel:any", from), but that was incorrect — from was never actually "camel:any", it was null. The test never passed after that change because the NPE on line 97 (from.toString()) would throw before the assertion was even reached.

With the null-safety fix, listTransformers() now returns "" for null DataTypes instead of throwing. So the assertion needs to match what actually comes back: "".

[github-actions]

🧪 CI tested the following changed modules:

• core/camel-management

ℹ️ Dependent modules were not tested because the total number of affected modules exceeded the threshold (50). Use the test-dependents label to force testing all dependents.

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