Fix scoped roadmap loading error and layout

What does this MR do and why?

Fixes a number of visual bugs around roadmap, both scoped (a view which opens from within an Epic) and full view;

• Loading the scoped view caused a JS error.
• Container height was miscalculated when there are too few epics to show.
• When applied filters on full roadmap view lead to empty results, settings sidebar was misaligned.

Screenshots or screen recordings

	Before	After
Scoped Roadmap (few epics)
Scoped Roadmap (several epics)
Settings Sidebar (empty state)
Settings Sidebar (with epics)

How to set up and validate locally

1. Go to a group
2. Create at least two epics with date range falling within current timeframe
3. Add one of the created epics as a child to one of the epic
4. Visit the epic which contains children
5. Click on Roadmap view button from Child issues and epics section
6. Make sure roadmap is loaded correctly without any related JS errors in browser console as well the layout is appearing properly

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.