Serialization error for a metric type containing null

[denhul]

When requesting "all" products via the GetAllAsync method, the method doesn't respond anymore after 2 requests per 100 items or after 13 requests per 10 items

[denhul]

@pardahlman it seems that the serialization to JSON can't handle null values for a metric type

Newtonsoft.Json.JsonSerializationException: Error converting value {null} to type 'System.Single'. Path 'amount', line 1, position 1396. ---> System.InvalidCastException: Null object cannot be converted to a value type.