[Bug fix] Append() in Nullable columns causes nulls_ array to be incorrectly appended to if nested column Append() fails

[moaazassali]

If Append() of the nested column fails, the nulls_ column will be incremented despite nothing being appended.

This causes the following issues: 1- The Size() reported by the Nullable<T> column will be wrong since it depends on the size of nulls_. 2- If you try to access this new "fake" element of the Nullable<T> column, an error will be thrown because false would be appended to nulls_ indicating that a valid non-null element exists. When At() or [] is used to access it, the actual nested column does not have this value, which causes the error to be thrown. 3- This is especially the case for columns like FixedString which could throw.

This pull request resolves that by moving the appending of nulls_ to the end of the Append() operation of the nested column. If the nested column Append() does fail, then the Append() to nulls_ is never called.

[Enmk]

Hi @moaazassali thank you for contribution! Please fix the issue mentioned in the review, and then we can merge it.