Remove duplicate :bigint entry and return nil for NULL Hive values

We must merge https://github.com/forward3d/rbhive/pull/22 in order to see the benefit of this change.

If the results from Hive are in their correct Ruby types, then the current code will call `conversion_method ? value.send(conversion_method) : value` to determine what value is returned to the result set. However, if value is nil, then calling the conversion_method on nil will create default values such as 0 and empty string.