Fix segment stats for layers without segment index

[dieknolle3333]

Steps to test:

• make sure you have a visible annotation layer without a fallback layer and without segment index, e.g. by viewing an old annotation (created before segment index was introduced) or by setting hasSegmentIndex to false in volumetracing_reducer
• brush and right-click a segment in the viewport. make sure there are not segment stats shown. also make sure that there is no Show Segment Statistics action in the context menu in the right border segment tab.
• click anywhere in the viewport where there is no segment and make sure the segment stats are not shown either

Issues:

• fixes https://scm.slack.com/archives/C5AKLAV0B/p1696856119961309

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(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Removed dev-only changes like prints and application.conf edits
• Considered common edge cases I tried, I swear 😅

[hotzenklotz]

LGTM

[hotzenklotz]