Handle NULL value for JSON data type

Author: mahadirz

pymysqlreplication/packet.py

@@ -343,6 +343,9 @@ def unpack_int32(self, n):
 
  def read_binary_json(self, size):
  length = self.read_uint_by_size(size)
+ if length == 0:
+ # handle NULL value
+ return None
  payload = self.read(length)
  self.unread(payload)
  t = self.read_uint8()