Guard against undefined metric.user in impact map (#8711)

MetricPointUserWithTooltip accesses metric.user.handle and
metric.user.avatarUrl, but user is optional on the Metric type.
Return an empty fragment when user data is missing.

Closes #8656

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: iHiD

app/javascript/components/impact/map.tsx

@@ -59,6 +59,8 @@ const MetricPointUserWithTooltip = ({
   metric: Metric
   text: string
 }): JSX.Element => {
+  if (!metric.user) return <></>
+
   const avatarRef = useRef(null)
   const content = (
     <div