Changed hll_add_agg() to return hll_empty instead of NULL

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 corresponding regression tests. (I didn't know if you'd like me to make changes to the documentation, so I left that as is.)