Python：如何处理CSV中的缺失值？

我有一个给定的CSV示例，如下所示：

ID,ID_TYPE,OB_DATE,VERSION_NUM,MET_DOMAIN_NAME,OB_END_CTIME,OB_DAY_CNT,SRC_ID,REC_ST_IND,PRCP_AMT,OB_DAY_CNT_Q,PRCP_AMT_Q,METO_STMP_TIME,MIDAS_STMP_ETIME,PRCP_AMT_J
90, RAIN, 2006-01-01 00:00,1, WADRAIN,900,1,24109,1011,0,0,6, 2006-01-17 09:04,0,
150, RAIN, 2006-01-01 00:00,1, DLY3208,900,1,30747,1011,0,0,6, 2006-01-09 13:21,3,
174, RAIN, 2006-01-01 00:00,1, WADRAIN,900,1,24775,1011,0.2,0,6, 2006-01-17 09:04,0,

我想在CSV中确定每个给定日期的工作日。我实现的代码如下所示：

import csv
from datetime import datetime as dt


csv_file = open('raindata.csv')
csv_reader = csv.DictReader(csv_file)
field_names = list(csv_reader.fieldnames)
if 'WEEKDAY' in field_names:
    print "data has error"

elif 'RECWEEKDAY' in field_names:
    print "data has error"

else:
    field_names.insert(field_names.index('OB_DATE') + 1, 'WEEKDAY')
    field_names.insert(field_names.index('METO_STMP_TIME') + 1, 'RECWEEKDAY')

    def get_weekday(ob_date):
        return dt.strptime(ob_date, ' %Y-%m-%d %H:%M').strftime('%A')

    output = open('raindata.csv','w')
    csv_writer = csv.DictWriter(output, field_names)
    csv_writer.writeheader()
    for row in csv_reader:
        row['WEEKDAY'] = get_weekday(row['OB_DATE'])
        row['RECWEEKDAY'] = get_weekday(row['METO_STMP_TIME'])
        csv_writer.writerow(row)

我的脚本可以正常运行并给出正确的结果，但是在OB_DATE列和METO_STMP_TIME列Date中缺少值的情况下，它将失败。

如何更改现有代码，以便对于空白Date值，相应Weekday值也为空白？