this solution is actually smart and clean

For readability, you should use Count for lists and Length for arrays. Arrays are always the same length. A list's count can change as you add items to it.

I did this with only arrays because I thought it said to return an array facepalm

Yes you are right. In real projects, it is better to check for NULL.

array is better, than list imho

Use spoiler flag next time, please.

What type does this actually return? A generic list or the query that finds the list?

Nice commenting. It helps understand the though behind the implementation.

I like the volumeSoFar and totalVolume names. Very clear.

I quite missunderstand your question but there are no checks just ariphmetics.