[DBAL-1001] Fix NULL / NOT NULL clause for column alterations in Oracle

Due to commit https://github.com/doctrine/dbal/commit/0782e9251a1df353ba589d32effbf75f646ca78f altering a column in Oracle now always appends a `NULL` / `NOT NULL` clause to the column alteration SQL which is wrong if the nullable status has not changed.
During column alteration the clause must only be appended if the nullable status really has changed.
See also: https://github.com/doctrine/dbal/pull/676#issuecomment-57643477