Add support for AddNullConstraint and DropNullConstraint to Firebird

Please add support for FirebirdDatabase:

• AddNullConstraint

• DropNullConstraint

Liquibase can do this using a simple update DML.

• Dropping null constraint

  update RDB$RELATION_FIELDS set RDB$NULL_FLAG = NULL where (RDB$FIELD_NAME = 'COLUMN_NAME') and (RDB$RELATION_NAME = 'TABLE_NAME')

• Adding null constraint

  update RDB$RELATION_FIELDS set RDB$NULL_FLAG = 1 where (RDB$FIELD_NAME = 'COLUMN_NAME') and (RDB$RELATION_NAME = 'TABLE_NAME')