Fix formatting product when variation has no attributes

Address #371

includes/class-wc-abstract-google-analytics-js.php

@@ -230,7 +230,7 @@ public function get_formatted_product( WC_Product $product, $variation = false )
 			$variation = $product->get_attributes();
 		}
 
-		if ( false !== $variation ) {
+		if ( false !== $variation && is_array( $variation ) ) {
 			$formatted['variation'] = implode(
 				', ',
 				array_map(