preprocess decorator: null_handler does not work

[coristig]

from yhat import Yhat, YhatModel , preprocess
import pandas as pd

class ReturnSentData(YhatModel):
    @preprocess(in_type=pd.DataFrame, out_type=dict, null_handler=[{"name":"key_2","na_filler":"No_value"}])
    def execute(self, data):
        return { "data_sent": data }

ReturnSentData().execute({"key_1": "value_1","key_2": ""})
Out[83]:
{'data_sent':      key_1 key_2
 0  value_1      }

It works below, but this is invalid json

ReturnSentData().execute({"key_1": "value_1","key_2": None})
Out[83]:
{'data_sent':      key_1     key_2
 0  value_1  No_value}