Fix non concatenation history observation handling case

[maks-bdai]

Change

• Moved concatenation check to beginning of function for early return
• Added a test to verify that the observation group with concat disabled works as expected

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there