对表的NOT NULL列约束强制执行不能将空值插入到表中。Flink支持“error”（默认）和“drop”强制行为 
The NOT NULL column constraint on a table enforces that null values can't be inserted into the table. Flink supports 'error' (default) and 'drop' enforcement behavior. By default, Flink will check values and throw runtime exception when null values writing into NOT NULL columns. Users can change the behavior to 'drop' to silently drop such records without throwing exception.
Possible values:
"ERROR" 
"DROP"