Add as possible return type of aggregate functions (#411)

[linusdm]

Add nil as possible return type ofo aggregate functions:

• sum
• min
• max
• mean
• median
• variance
• standard_deviation
• quantile

Each one of these functions can return nil when invoked with an empty Series.

[linusdm]

I wasn't able to compile the project locally, so I'm not sure if all tests still run. I've tried all aggregation functions listed, to see whether they actually return nil.

[philss]

@linusdm thank you! 💜