QSqlTableModel::setData() needs to compare isNull() when comparing old and new QVariant value

Consider the table below:

CREATE TABLE IF NOT EXISTS collection (
  id INTEGER PRIMARY KEY AUTOINCREMENT,
  parent_id INTEGER,
  order_num INTEGER NOT NULL
)

When I insert QSqlRecord built with values fetched from QTextEdit's, i.e. pair of strings (parent_id, order_num), I end up with the following results:

• ("", "0") — QSqlError(-1, "No Fields to update", "")
• ("", "1") — No error. Row is actually inserted
• ("0", "0") — QSqlError(-1, "No Fields to update", "")
• ("1", "0") — QSqlError(19, "Unable to fetch row", "collection.order_num may not be NULL")

See attachment test2.zip for the minimal demo.