Add null guard for valueData in AttributesFilter computed properties

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/Discover/Map/Filter/AttributesFilter.vue

@@ -435,6 +435,7 @@ const valueData = computed(() => {
 
 // Compute slider data; time format: millisecond timestamps
 const sliderDataComputed = computed(() => {
+  if (!valueData.value) return [];
   let xAxisMin;
   let xAxisMax;
   const min = valueData.value.min;
@@ -511,6 +512,7 @@ const attValueTypeStartEnd = computed(() => {
 });
 
 const labelsData = computed(() => {
+  if (!valueData.value) return [];
   const min = new Date(valueData.value.min).getTime();
   const max = new Date(valueData.value.max).getTime();
   const numPoints = 11;