fix: go client omit optional scope param if not set

[megglos]

Description

The optional scope parameter was always sent with an empty string value on token request, if not explicitly set. This caused the token request to fail as either none or a valid scope needs to be provided.

Related issues

closes #16015

[megglos]

Validated manually with a local build of zbctl as well:

./zbctl.darwin --address=localhost:26500 --clientId=zeebe --clientSecret=zecret --audience=zeebe-api --authzUrl=http://localhost:18080/auth/realms/camunda-platform/protocol/openid-connect/token --insecure status
2024/05/02 07:21:05 Warning: The configured security level does not guarantee that the credentials will be confidential. If this unintentional, please enable transport security.
Cluster size: 1
Partitions count: 1
Replication factor: 1
Gateway version: 8.4.6
Brokers:
  Broker 0 - 172.26.0.6:26501
    Version: 8.4.6
    Partition 1 : Leader, Healthy

[backport-action]

Successfully created backport PR for stable/8.4:

• 18155

[backport-action]

Successfully created backport PR for stable/8.5:

• 18156