[2.5] If an attribute meta key is not set, technically its 'any', so should match. Prevents issues when meta data is missing after renaming attributes

@claudiosmweb please test :)

#10272

includes/class-wc-product-variable.php

@@ -510,10 +510,18 @@ class WC_Product_Variable extends WC_Product {
 			$value = wc_clean( $match_attributes[ $attribute_field_name ] );
 
 			$query_args['meta_query'][] = array(
-				'key'     => $attribute_field_name,
-				'value'   => array( '', $value ),
-				'compare' => 'IN'
+				'relation' => 'OR',
+				array(
+					'key'     => $attribute_field_name,
+					'value'   => array( '', $value ),
+					'compare' => 'IN'
+				),
+				array(
+					'key'     => $attribute_field_name,
+					'compare' => 'NOT EXISTS'
+				)
 			);
+
 		}
 
 		$matches = get_posts( $query_args );