Fix calculation of AVERAGE when null values are present

[OlgaGorlova]

When there are null values, the AVERAGE function gives a wrong result - it uses count of all values including nulls. Instead, it should count only values without nulls.

[marcomass]

@OlgaGorlova Good catch. Does this apply only on AVERAGE or also on other aggregation functions? Did you test all of them (MIN, MAX, MEDIAN, STD, BAG, ...)?

[OlgaGorlova]

@marcomass, should be only on AVERAGE

[marcomass]

@OlgaGorlova Does it mean that you tested all the other aggregation functions and they work correctly also in presence of nulls?

[OlgaGorlova]

@marcomass Yes, I tested them and they are working ok, but I did it on a small synthetic data.

[Erlaad]

Isn't this a partial duplicate of issue #14 ? If so, I tested and found it working (and reported on related issue).