Compacting an empy index creates errors.

When you compacted an index that didn't contain any data,
GeoCouch spit out a huge stack trace. This commit fixes
the problem.

Change-Id: I201683ea135bf00841734595024e8c7e436292f0
Reviewed-on: http://review.couchbase.org/9781
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Dale Harvey <daleharvey@arandomurl.com>
Reviewed-by: Dale Harvey <daleharvey@arandomurl.com>

src/geocouch/couch_spatial_compactor.erl

@@ -77,7 +77,11 @@ compact_group(Group, EmptyGroup) ->
     {ok, NewIdBtree} = couch_btree:add(Bt3, lists:reverse(Uncopied)),
 
     NewIndexes = lists:map(fun({Index, EmptyIndex}) ->
-        compact_spatial(Fd, EmptyFd, Index, EmptyIndex)
+        case Index#spatial.treepos of
+            % Tree is empty, just grab the the FD
+            nil -> EmptyIndex#spatial{fd = EmptyFd};
+            _ -> compact_spatial(Fd, EmptyFd, Index, EmptyIndex)
+        end
     end, lists:zip(Indexes, EmptyIndexes)),
 
     NewGroup = EmptyGroup#spatial_group{