Move ?? operator from get_source_values back to `get_unprefixed_f…

…ields`

 because `$_POST` may not include all these values.

Co-authored-by: Justin Palmer <228780+layoutd@users.noreply.github.com>

plugins/woocommerce/src/Internal/Traits/OrderAttributionMeta.php

@@ -244,7 +244,7 @@ private function get_unprefixed_fields( array $raw_values = array() ): array {
 
 		// Look through each field in POST data.
 		foreach ( $this->fields as $field ) {
-			$values[ $field ] = $raw_values[ $this->get_prefixed_field( $field ) ];
+			$values[ $field ] = $raw_values[ $this->get_prefixed_field( $field ) ] ?? '(none)';
 		}
 
 		return $values;
@@ -262,7 +262,7 @@ private function get_source_values( array $raw_values = array() ): array {
 
 		// Look through each field in given data.
 		foreach ( $this->fields as $field ) {
-			$value = sanitize_text_field( wp_unslash( $raw_values[ $field ] ?? '(none)' ) );
+			$value = sanitize_text_field( wp_unslash( $raw_values[ $field ] ) );
 			if ( '(none)' === $value ) {
 				continue;
 			}