Check for existence of `footer/script-footer-core` partial

[salim-b]

Summary

Check whether file exists before calling partial footer/script-footer-core.html.

Checks

• [x] Read Create a Pull Request
• [x] Supports all screen sizes (if relevant)
• [x] Supports both light and dark mode (if relevant)
• [ ] Passes npm run test

[salim-b]

Sorry, first didn't realize that the footer/script-footer-core partial actually does exist in hyas-core. I've updated the PR accordingly.