Can't add a stage to a nested query

[christopherswenson]

1. Nest a query
2. Duplicate it (unless you created the query with "New Nested Query")
3. Try to add a stage to it
4. Boom

[christopherswenson]

This has something to do with attempting to generate a query string for a query with an empty stage, and failing when we ask the compiler to tell us what the source for the stage is. We might be able to skip that step when we realize the stage is empty...