Running Thicket.concat_thickets with calltree="intersection" produces empty query

When trying to merge 4 Thickets with RAJAPerf data with `concat_thickets`, Thicket errors with `EmptyQuery` because the query used in `Thicket.intersection` would produce an empty Thicket object. After reviewing the individual Thickets by hand, I found that their call trees are identical, so there is a bug in `Thicket.intersection` causing this error.

Reproduce the failure by merging four RAJAPerf Thickets with concat_thickets and calltree="intersection". Start by tracing Thicket.intersection and the query that raises EmptyQuery; compare its result with the individual call trees. Done means identical call trees produce a non-empty Thicket and concat_thickets completes without EmptyQuery.

Written by the indexing model from the issue text.