How to set value to NULL in editable QTableView with QSqlTableModel

I have a QTableView that displays subset of the data from a table in SQLite database. The table is editable only for nullable numeric columns. I created two delegates - one for read-only columns:

class ReadOnlyDelegate(QtWidgets.QItemDelegate):
    def editorEvent(self, *args, **kwargs):
        return False

    def createEditor(self, *args, **kwargs):
        return None

and one for editable columns:

class LabelDelegate(QtWidgets.QItemDelegate):
    def createEditor(self, parent, options, index):
        self.le = QtWidgets.QLineEdit(parent)
        return self.le

The table is fed by customized QSqlTableModel, where I overwrite submitAll method:

class MySqlTableModel(QtSql.QSqlTableModel):
    def submitAll(self):
        for row in range(self.rowCount()):
            for col in range(self.columnCount()):
                if self.isDirty(self.index(row, col)):
                    val = self.record(row).value(col)
                    if val == '':
                        self.record(row).setNull(col)
                    else:
                        try:
                            self.record(row).setValue(col, float(val))
                        except (TypeError, ValueError):
                            display_error_msg('Can not convert to float',
                                              f'The value {val} could not be converted to float')
                            raise
        super().submitAll()

Expected behaviour is (1) to convert values to float before sending to database, (2) reject inputs that can't be converted to float and (3) to convert empty string to NULL. (1) and (2) work as expected, however the last bit is not working. When debugging method .submitAll() it raises no exception on the line self.record(row).setNull(col) but it also seems to have no effect. An empty string is sent and persisted in database. Any ideas why and how to fix it?

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