cln-grpc: Skip serializing fields when Option<Vec<T>> is empty too

The CLN API is rather strict about the fact that we should skip
providing a field whenever it is empty. Checking for `is_none` would
still include empty arrays.

Changelog-Fixed cln-rpc: Optional empty arrays will no longer be serialized in requests

cln-rpc/src/lib.rs

@@ -106,6 +106,16 @@ impl ClnRpc {
     }
 }
 
+/// Used to skip optional arrays when serializing requests.
+fn is_none_or_empty<T>(f: &Option<Vec<T>>) -> bool
+where
+    T: Clone,
+{
+    // TODO Find a better way to check, possibly without cloning
+    let f =    f.clone();
+    f.is_none() || f.unwrap().is_empty()
+}
+
 #[cfg(test)]
 mod test {
     use super::*;