kernels-data: skip serializing archs when None (#493)

Author: danieldk

kernels-data/src/metadata.rs

@@ -10,6 +10,7 @@ use crate::config::Backend;
 pub struct BackendInfo {
     #[serde(rename = "type")]
     pub backend_type: Backend,
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub archs: Option<Vec<String>>,
 }