fix column ddl bug

[kgpayne]

Column.table can be None, and when that happens it results in:

AttributeError: 'NoneType' object has no attribute '_autoincrement_column'

We use SQLAlchemy to compose dialect-correct DDL statements like:

import sqlalchemy as sa
from sqlalchemy.sql import ddl

create_column_clause = ddl.CreateColumn(
    sa.Column(
       column_name,
        column_type,
    ),
)
compiled = create_column_clause.compile(self._engine).string
return sa.DDL(
    "ALTER TABLE %(table_name)s ADD COLUMN %(create_column_clause)s",
    {
        "table_name": table_name,
        "create_column_clause": compiled,
    },
)

This is why Column.table is None.