avoid create credentials related to empty groups

[piif]

Description

Playbook warns if there are missing groups like "schema-registry" or "kafka-rest", thus we define them with no entries.

Because the groups exist, playbook generate useless credentials to related users

Type of change

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

Tested with empty groups in inventory

Checklist:

• [ ] Any variable/code changes have been validated to be backwards compatible (doesn't break upgrade)
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] If required, I have ensured the changes can be discovered by cp-ansible discovery codebase
• [ ] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [ ] Any dependent changes have been merged and published in downstream modules