get_control_response missing "This System" component

[rahearn]

Describe the bug

For controls with statements, the "This System" implementation statements are missing from the individual parts.

This appears to be a regression introduced in #1358

To Reproduce

Steps to reproduce the behavior:

1. Create a system security plan with a control with multiple parts (such as SC-7)
2. Use trestle author jinja in combination with ssp_md_writer.get_control_response to create a markdown SSPP
3. See that the implementation statements for each individual part are missing for the This System component (though they'll be present for any other components)

Expected behavior

I expected that This System implementation statements are included for all parts of the control.

Screenshots / Logs.

If applicable, add screenshots to help explain your problem.

Environment

• OS: [e.g. iOS] Docker/Debian
• Python version: 3.11.11
• Installed packages: compliance-trestle 3.8.0