unmarshall nil and zero decimal into inf.Dec unsupported

[illia-li]

Currently if we try unmarshalls decimal with nil and zero value into inf.Dec - the unmarshall function return an error: inf.Dec needs at least 4 bytes, while value has only 0
According to cql protocol - for all types value can be nil and zero.
Therefore unmarshall function should support unmarshalls nil and zero value for all types.