fix(2meet-data-optimizer): do not recreate a deleted entity's flat row

- Deleting a post / user / term / comment drops its flat row in the adapter delete hook, then WordPress deletes each meta row; sync_from_native upserted NULL for each and brought the row back (512 orphan rows on dev36). With no native value it now only clears the column on an existing row
- Verify: tests/live/run.php resync (25/25, entity deletes in dual_write and shadow_read leave no flat row); mut.php T8 flat_row NO-ROW; integration 516
- PR: PR-057b

tests/integration/HookBusIntegrationTest.php

@@ -256,6 +256,24 @@ class HookBusIntegrationTest extends TestCase {
 		);
 	}
 
+	/**
+	 * Deleting an entity drops its flat row first (adapter delete hook), then
+	 * WordPress deletes each meta row; syncing those deletions must not bring
+	 * the row back as all NULL.
+	 */
+	public function test_sync_of_a_deleted_key_does_not_recreate_a_missing_row(): void {
+		global $wpdb;
+		self::set_user_mode( WPDO_Mode_Manager::MODE_DUAL_WRITE );
+
+		$this->with_usermeta(
+			function () use ( $wpdb ) {
+				WPDO_Hook_Bus::sync_from_native( 'user', 31, 'first_name', 'Gone', 'delete' );
+
+				$this->assertSame( '0', $wpdb->get_var( 'SELECT COUNT(*) FROM `' . self::FLAT_TABLE . '` WHERE user_id = 31' ) );
+			}
+		);
+	}
+
 	/**
 	 * Run $fn with a native usermeta table, dropped afterwards.
 	 */