
Treat `nil` Amount as 0 in `resource.Quantity.Add` and `resource.Quantity.Sub`. Also, allow adding/subtracting resources with different Formats (since Format has no effect on the underlying value).
Treat `nil` Amount as 0 in `resource.Quantity.Add` and `resource.Quantity.Sub`. Also, allow adding/subtracting resources with different Formats (since Format has no effect on the underlying value).