Execute Query without ->from()

[Tigrov]

Executing Query without FROM statement fails in Oracle:

$scalar = (new Query($db))->select(new Expression('1'))->scalar();

Tests yiisoft/db#760

https://github.com/yiisoft/db/actions/runs/6324733173/job/17174800505?pr=760

1) Yiisoft\Db\Oracle\Tests\QueryTest::testSelectWithoutFrom
Yiisoft\Db\Exception\IntegrityException: SQLSTATE[HY000]: General error: 923 OCIStmtExecute: ORA-00923: FROM keyword not found where expected
 (/tmp/php-src-php-8.2.10/ext/pdo_oci/oci_statement.c:153)
The SQL being executed was: SELECT 1

This can be solved by adding FROM DUAL for SQL expressions without FROM statement.