[Cosmos] Returns undefined for cross-partition sums when one is undefined

[zfoster]

We had an issue noticed in the portal where we were returning seemingly correct values for queries like

SELECT VALUE SUM(c.age) from c

for cross-partition queries, even when age had null or undefined values. For example, with the following items, we were returning the value 44

{ id: id1, age: 22 },
{ id: id2, age: 22 },
{ id: id3, age: null },
{ id: id4, age: 22 },
{ id: id5 },
{ id: id6 }

which is inconsistent with other SDKs

[zfoster]

new PR coming