Let an empty result be an answer, not an abort

worker.table([], { index: 'pf_gkey' }) aborts: an empty array carries no
columns, so the index names one that does not exist and the page dies with
"Specified index `pf_gkey` does not exist in dataset" instead of saying it
found nothing.

Nothing is a legitimate answer -- an empty version, and now a territory with
no rows in it, which is what surfaced this. The empty table is built without
an index and the page says which of the two it is.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/src/views/Forecast.jsx

@@ -777,8 +777,18 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
         if (stale) await stale.delete()
       } catch {}
 
-      const opts = { name: tableName, index: indexCol }
+      // An empty result gets no index. `[]` carries no columns, so naming one
+      // aborts the worker outright -- "Specified index `pf_gkey` does not exist
+      // in dataset" -- and the page dies rather than saying it found nothing.
+      // Nothing is a legitimate answer: an empty version, or a territory with no
+      // rows in it.
+      const opts = rowCount > 0
+        ? { name: tableName, index: indexCol }
+        : { name: tableName }
       tableRef.current = await (rowCount > 0 ? worker.table(buffer, opts) : worker.table([], opts))
+      if (rowCount === 0) {
+        flash('No rows to show — the version is empty, or none of it is in your territory', 'error')
+      }
 
       if (myId !== initIdRef.current) {
         try { await tableRef.current.delete() } catch {}