Make Numeric.sum return null when there are no inputs or all null inputs

[deephaven-internal]

This issue was auto-generated

PR: deephaven/deephaven-core#5461
Author: chipkent

Original PR Body

Make Numeric.sum return null when there are no inputs or all null inputs. This makes the method consistent with the aggregations.

[jjbrosnan]

This ticket should require zero docs work. We don't document this anywhere, and it's just a fix, not an actual change to the API.