Impossible to update imported topics config

[ThomasCAI-mlv]

Describe the bug

When a topic is created outside of ns4kafka, then is imported to ns4kafka with POST /api/namespaces/<namespace>/topics/_/import, this topic config (retention.ms, min.insync.replicas for example) cannot be updated with POST /api/namespaces/<namespace>/topics

Notes:

1. The issue was found when the topic is created on Confluent Cloud. I don't know yet if it applies for self-hosted kafka clusters.
2. We have the same issue when we try the delete-records endpoint on imported topics.

To Reproduce

1. Create a namespace "myNamespace" with owner ACL on topic prefixed by "myNamespace."
2. On Confluent Cloud, manually create a new topic "myNamespace.myTopic" with the default configurations.
3. With ns4kafka, import the topics with the endpoint POST /api/namespaces/myNamespace/topics/_/import
4. With ns4kafka, try to update the topic config with the endpoint POST /api/namespaces/myNamespace/topics and the following body:

{
	"apiVersion": "v1",
	"kind": "Topic",
	"metadata": {
		"name": "myNamespace.myTopic"
	},
	"spec": {
		"replicationFactor": 3,
		"partitions": 1,
		"configs": {
			"min.insync.replicas": "3",
			"cleanup.policy": "delete",
			"retention.ms": "86400000"
		}
	}
}

The API call failed with 500 Internal Servor Error: Cannot invoke \"String.equals(Object)\" because the return value of \"java.util.Map.get(Object)\" is null

Environment:

• Kafkactl distribution and version: not relevant
• Ns4Kafka version: v1.14.0
• Kafka broker type and version Apache Kafka 3.8 / Confluent Platform 7.8

Steps taken to try and solve the problem

To bypass delete-records, the topic can be deleted then recreated through ns4kafka. But for topic update, I don't think we have any workaround except fixing the bug in ns4kafka directly.