Add filter empty state to milestone list

What does this MR do and why?

Adds filter empty state to milestone list

Splits empty state between no items, and no filter results, to clarify and align with Pajamas.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
		No filter match
		No items in tab (no change)
		No milestones (no change)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Open a group or project and navigate to the milestone list
2. Apply a search that won't match any items to observe the filter empty state
3. Close all milestones (or open a group/project with no open milestones, but with closed milestones) to observe the empty state based on tab selection
4. Open a group or project with no milestones to observe the empty state with no items

Related to #480851 (closed)