- Use the COLDEF flag to initialize column nullable value.
- Fix a bug on inserting null values in MYSQL tables.

modified:
  storage/connect/colblk.cpp
  storage/connect/connect.cc
  storage/connect/ha_connect.cc
  storage/connect/odbconn.cpp
  storage/connect/plgdbsem.h
  storage/connect/table.cpp
  storage/connect/tabmysql.cpp