**Description**
The change addNotNullConstraint and dropNotNullConstraint are not available for Firebird databases.

**To Reproduce**
Add the commands to a change set and run it against a Firebird database. As a result there will be the following error messages:
_addNotNullConstraint is not supported on firebird_
_dropNotNullConstraint is not supported on firebird_

**Expected behavior**
The commands should be available for Firebird databases as well.


} else if (database instanceof MySQLDatabase) {
            sql = "ALTER TABLE " + database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) + " MODIFY " + database.escapeColumnName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName(), statement.getColumnName()) + " " + DataTypeFactory.getInstance().fromDescription(statement.getColumnDataType(), database).toDatabaseDataType(database) + nullableString;
        } else if (database instanceof DerbyDatabase) {
            sql = "ALTER TABLE " + database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) + " ALTER COLUMN  " + database.escapeColumnName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName(), statement.getColumnName()) + nullableString;
        } else if ((database instanceof HsqlDatabase) || (database instanceof H2Database)) {
            sql = "ALTER TABLE " + database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) + " ALTER COLUMN " + database.escapeColumnName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName(), statement.getColumnName()) + " SET"+nullableString;
        } else if (database instanceof InformixDatabase) {
            
            if (statement.isNullable()) {
                nullableString = "";
            }
            sql = "ALTER TABLE " + database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) + " MODIFY (" + database.escapeColumnName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName(), statement.getColumnName()) + " " + DataTypeFactory.getInstance().fromDescription(statement.getColumnDataType(), database).toDatabaseDataType(database) + nullableString + ")";
        } else {
            sql = "ALTER TABLE " + database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) + " ALTER COLUMN  " + database.escapeColumnName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName(), statement.getColumnName()) + (statement.isNullable() ? " DROP NOT NULL" : " SET NOT NULL");
        }
        List<Sql> returnList = new ArrayList<>();
        returnList.add(new UnparsedSql(sql, getAffectedColumn(statement)));
        if (database instanceof DB2Database) {
            Sql[] a = SqlGeneratorFactory.getInstance().generateSql(new ReorganizeTableStatement(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()), database);
            if (a != null) {
                returnList.addAll(Arrays.asList(a));
            }
        }
        return returnList.toArray(new Sql[returnList.size()]);
    }
    protected Column getAffectedColumn(SetNullableStatement statement) {
        return new Column().setName(statement.getColumnName()).setRelation(new Table().setName(statement.getTableName()).setSchema(statement.getCatalogName(), statement.getSchemaName()));
    }
}