Fix Redpanda global config compatibility

[reibengu]

This project was not correctly working with Redpanda schema registry because it incorrectly builds the config path by adding an extra slash.

Here is the error I got:

2025-10-16 12:10:54.372 DEBUG Sending GET with input null to https://....prd.cloud.redpanda.com/config/?defaultToGlobal=false
2025-10-16 12:10:54.515 ERROR io.confluent.kafka.schemaregistry.client.rest.exceptions.RestClientException: Unable to parse error message from schema registry: '(404 page not found)'; error code: 50005

Ref on how the client builds this url: https://github.com/confluentinc/schema-registry/blob/master/client/src/main/java/io/confluent/kafka/schemaregistry/client/rest/RestService.java#L1003 and why passing null is better than empty string.