Handling of nulls with Number differs from encoding/json

[oschwald]

This is similar to #181, but for null. encoding/json unmarshals a null value to json.Number("") while jsoniter returns an error:

readNumberAsString: invalid number, parsing 53 ..."", "t5": ... at {"test":"blah", "t2":472, "t3":"473", "t4":"", "t5": null}