pageserver: handle empty get vectored queries (#11652)

## Problem

If all batched requests are excluded from the query by
`Timeine::get_rel_page_at_lsn_batched` (e.g. because they are past the
end of the relation), the read path would panic since it doesn't expect
empty queries. This is a change in behaviour that was introduced with
the scattered query implementation.

## Summary of Changes

Handle empty queries explicitly.

pageserver/src/tenant/timeline.rs

@@ -1285,6 +1285,10 @@ impl Timeline {
         reconstruct_state: &mut ValuesReconstructState,
         ctx: &RequestContext,
     ) -> Result<BTreeMap<Key, Result<Bytes, PageReconstructError>>, GetVectoredError> {
+        if query.is_empty() {
+            return Ok(BTreeMap::default());
+        }
+
         let read_path = if self.conf.enable_read_path_debugging || ctx.read_path_debug() {
             Some(ReadPath::new(
                 query.total_keyspace(),