migurski/TileStache — pull request #146 by jwass (2013-07-30 02:36:34+00:00)
Title: Return empty response for queries with no features

If query_columns() finds no features, the columns can't be determined causing
a crash. Explicitly return None for the columns and generate an
EmptyResponse.

I found this bug when I had a typo in a query causing no features to be found in query_columns as it zooms out looking for them. I figured returning an EmptyResponse rather than raising an exception is the right thing here.