**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.


return false;
            }
        } catch (DatabaseException ignore) {
            
        }
        return !((database instanceof FirebirdDatabase) || (database instanceof SQLiteDatabase));
    }
    @Override
    public ValidationErrors validate(SetNullableStatement setNullableStatement, Database database, SqlGeneratorChain sqlGeneratorChain) {
        ValidationErrors validationErrors = new ValidationErrors();
        validationErrors.checkRequiredField("tableName", setNullableStatement.getTableName());
        validationErrors.checkRequiredField("columnName", setNullableStatement.getColumnName());
        if ((database instanceof MSSQLDatabase) || (database instanceof MySQLDatabase) || (database instanceof InformixDatabase)) {
            validationErrors.checkRequiredField("columnDataType", setNullableStatement.getColumnDataType());
        }
        return validationErrors;
    }
    @Override
    public Sql[] generateSql(SetNullableStatement statement, Database database, SqlGeneratorChain sqlGeneratorChain) {
        String sql;
        String nullableString = statement.isNullable()?" NULL":" NOT NULL";
        if ((database instanceof OracleDatabase) && (statement.getConstraintName() != null)) {
            nullableString += !statement.isValidate() ? " ENABLE NOVALIDATE " : "";
            sql = "ALTER TABLE " + database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) + " MODIFY " + database.escapeColumnName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName(), statement.getColumnName()) + " CONSTRAINT " + statement.getConstraintName() + nullableString;
        } else if ((database instanceof OracleDatabase) || (database instanceof SybaseDatabase) || (database
            instanceof SybaseASADatabase)) {
            nullableString += (database instanceof OracleDatabase)&&(!statement.isValidate()) ? " ENABLE NOVALIDATE " : "";
            sql = "ALTER TABLE " + database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) + " MODIFY " + database.escapeColumnName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName(), statement.getColumnName()) + nullableString;
        } else if (database instanceof MSSQLDatabase) {
            sql = "ALTER TABLE " + database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) + " ALTER COLUMN " + database.escapeColumnName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName(), statement.getColumnName()) + " " + DataTypeFactory.getInstance().fromDescription(statement.getColumnDataType(), database).toDatabaseDataType(database) + nullableString;