Add assert that pruned list is non-empty

If DistributedQueryShardList ever returns an empty list, it's because
no shards exist that can satisfy the given query. This shouldn't be
possible if everything is working correctly, so we want an Assert here
instead of error checking.

pg_shard.c

@@ -563,6 +563,9 @@ DistributedQueryShardList(Query *query)
 	prunedShardList = PruneShardList(distributedTableId, restrictClauseList,
 									 shardIntervalList);
 
+	/* shouldn't be an empty list, but assert in case something's very wrong */
+	Assert(prunedShardList != NIL);
+
 	return prunedShardList;
 }