nullify server object on shutdown

[nate-nutmeg]

Nulls the server on shutdown to allow future tests to create a new server instance.

Checklist

• tested locally
• added new dependencies
• updated the docs
• added a test

[fnobilia]

Thx for the PR @nate-nutmeg!

Can we add two more tests showing that

1. a flow of start, stop and start will provide access to the same schemata in case the Kafka cluster is not cleaned it up
2. a flow of start, stop and start will not provide access to the same schemata in case the Kafka cluster is cleaned it up

[nate-nutmeg]

I am not sure about SchemaRegistryTestResourceLifecycleTest now that we have created it.

I feel that we might be testing more than schemaregistry-junit at this point. The test shouldBeAbleToAccessSchemasAfterSchemaRegistryTestResourceRestart is basically testing that schema-registry is able to read schemas from the _schemas topic. This is testing the functionality of schema-registry over testing the functionality of schemaregistry-junit.

Similarily with the next test. Testing that we can't access schemas after deleting the _schemas topic on a new SchemaRegistryTestResource. This feels more like testing schema-registry again. I can't get this to work strangely though. But none of the code is ours.

The last test shouldNotBeAbleToAccessSchemasOnEmptySchemasTopicWithSeperateKafkaInstances uses a new Kafka instance. It uses a hack of passing a null ExecutionContext to the beforeAll. But this just tests that the schema-registry is able to create a _schemas topic on a new kafka cluster twice in one test. I not sure it has much bearing on testing SchemaRegistryTestResource

I will leave it up to you if you want to keep the test

[fnobilia]

@nate-nutmeg I think we are not testing the Schema Registry source code, but we are testing that we don't do anything more than starting or stopping the server when calling start or stop functions. Tests are showing that we need to alter Kafka Cluster state to change Schema Registry storage behaviour.

Let's fix the compilation issue and then it is good to go! 🚀

Thx again for the contribution! ☺

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[fnobilia]

The issue with Sonarcloud cannot be solved. I tried multiple solutions suggested in a couple of Stackoverflow, but no luck. Regression testing, example tests and tests on the main code are working. I'll take the "risk" to merge into master without Sonarcloud passing.

Thx @nate-nutmeg for your contribution.