**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.


package liquibase.sqlgenerator.core;
import liquibase.database.Database;
import liquibase.database.core.*;
import liquibase.datatype.DataTypeFactory;
import liquibase.exception.DatabaseException;
import liquibase.exception.ValidationErrors;
import liquibase.sql.Sql;
import liquibase.sql.UnparsedSql;
import liquibase.sqlgenerator.SqlGeneratorChain;
import liquibase.sqlgenerator.SqlGeneratorFactory;
import liquibase.statement.core.ReorganizeTableStatement;
import liquibase.statement.core.SetNullableStatement;
import liquibase.structure.core.Column;
import liquibase.structure.core.Table;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class SetNullableGenerator extends AbstractSqlGenerator<SetNullableStatement> {
    @Override
    public boolean supports(SetNullableStatement statement, Database database) {
        try {
            if (database instanceof Db2zDatabase || (database instanceof DB2Database) && (database.getDatabaseMajorVersion() > 0 && database.getDatabaseMajorVersion() < 9)) {