be explicit over nullable fields in GDN_MySQLStructure

Mark columns as explicitly null when they are marked as such in the structure

Per the MySQL column definition this is perfectly fine:

```
column_definition:
    data_type [NOT NULL | NULL] [DEFAULT default_value]
      [AUTO_INCREMENT] [UNIQUE [KEY] | [PRIMARY] KEY]
      [COMMENT 'string']
      [COLUMN_FORMAT {FIXED|DYNAMIC|DEFAULT}]
      [STORAGE {DISK|MEMORY|DEFAULT}]
      [reference_definition]
```