fix: query() returned null instead of 0 for statements affecting 0 rows

[chloe-pomegranate]

Summary

Fixes #322

WP_SQLite_Driver::query() returned null instead of 0 for statements affecting zero rows (e.g., DELETE FROM x WHERE id = 1 when no matching row exists). This caused wpdb::query() to also return null, breaking its documented contract of returning int|bool.

Problem

In WP_SQLite_Driver::query(), the return logic for non-SELECT queries was:

1. rowCount() > 0 - return row count
2. Otherwise - return null

This meant any DELETE, UPDATE, INSERT statement with 0 affected rows returned null instead of 0.

Fix

Simplified the logic to always return rowCount() for non-SELECT statements.

DELETE / UPDATE / INSERT will now correctly returns 0 instead of null when no rows affected.
CREATE / ALTER / DROP / TRUNCATE now returns 0 instead of null, but wp-includes/sqlite/class-wp-sqlite-db.php never reads this value for these queries (it returns true unconditionally at line 448-449)

Testing

• Added testDeleteReturnsZeroAffectedRowsWhenNoMatchingRows - verifies DELETE returns 0 when no rows match.
• Added testUpdateReturnsZeroAffectedRowsWhenNoMatchingRows - verifies UPDATE returns 0 when no rows match.
• Updated existing test assertions from assertNull to assertSame( 0, ... ) to reflect the corrected behaviour
• Full test suite passes (777 tests)