更新字段默认null

Model/models.py

@@ -3737,8 +3737,8 @@ class IotCardUsageHistory(models.Model):
     flow_total_usage = models.DecimalField(default=0, max_digits=10, decimal_places=2, verbose_name=u'月流量用量(MB)')
     voice_total_usage = models.IntegerField(default=0, verbose_name='帐期已用语音总量（Min）')
     sms_total_usage = models.IntegerField(default=0, verbose_name='帐期已用短信总量（条）')
-    cycle_start_time = models.DateTimeField(blank=True, verbose_name='帐期开始时间')
-    cycle_end_time = models.DateTimeField(blank=True, verbose_name='帐期结束时间')
+    cycle_start_time = models.DateTimeField(null=True, blank=True, verbose_name='帐期开始时间')
+    cycle_end_time = models.DateTimeField(null=True, blank=True, verbose_name='帐期结束时间')
     created_time = models.IntegerField(default=0, verbose_name='创建时间')
     updated_time = models.IntegerField(default=0, verbose_name='修改时间')