Changed hll_add_trans() functions to return hll_empty instead of NULL when aggregating over empty sets. This makes hll_add_agg() conform to PostgreSQL's count() semantics over empty sets, and closes issue #2. Also added two small regression tests.
Added a few more regression tests for the previous hll_add_trans() changes.
Merge pull request #11 from ozgune/master

Changed hll_add_agg() to return hll_empty instead of NULL.