return hll_empty for no input rows in hll_add_agg

[beyzaaydogan]

When I debugged Issue #129, I observed that the state transition function, hll_add_trans, was not being called at all. This was because the values used in the filter did not satisfy the condition. However, I noticed that the final function, hll_pack, was called with a null input. In this case, I made development so that if hll_pack is called with a null input, it will return hll_empty.

hll_pack is the final function for hll_union_agg as well. With my fix, if hll_pack is called with a null parameter while using hll_union_agg, it returns hll_empty. This is causing the failure of this test:

SELECT hll_union_agg(v1) FROM test_wdflbzfx WHERE recno > 100;

Moreover, when hll_add_agg is directly called with null parameters, it also returns hll_empty, as mentioned in Issue #2. However, hll_union_agg returns null in such a case. Shouldn't hll_union_agg also return hll_empty when called with null parameters? What are your thoughts on this matter?