receipts/add: Do not query Firefly for empty ID

The `transaction` field is usually included in the form submission, even
if it is empty.  To avoid querying Firefly for an invalid ID and logging
a useless error about it, we only query if the field is non-empty.

src/routes/receipts.rs

@@ -113,7 +113,11 @@ pub async fn add_receipt(
                 },
             }
         },
-        Some(ref id) => match ctx.firefly.get_transaction(id).await {
+        Some(ref id) if !id.is_empty() => match ctx
+            .firefly
+            .get_transaction(id)
+            .await
+        {
             Ok(t) => {
                 let mut needs_update = false;
                 let mut update = TransactionUpdate::from(t.data.clone());
@@ -166,6 +170,7 @@ pub async fn add_receipt(
                 None
             },
         },
+        Some(_) => None,
         None => None,
     };
     if let Some(xact) = xact {