Fix change detection: handle empty memo correctly

memo.to_utf8() returns Some(Ok("")) for change outputs with all-zero
bytes, not None. The previous check (memo.to_utf8().is_none()) never
filtered change outputs because 0x00 < 0xF5 makes to_utf8() treat it
as valid text. Now properly checks for empty string as well.

lib/src/lightwallet.rs

@@ -1482,10 +1482,17 @@ pub fn scan_full_tx(&self, tx: &Transaction, height: i32, datetime: u64) {
                             try_sapling_note_decryption(ivk, &epk_prime, &output.cmu, &output.enc_ciphertext).is_some()
                         });
 
-                        // If this is change (funds to ourself) without a memo, skip it.
-                        // If the user sent a memo to themselves, keep it in outgoing metadata.
-                        if is_to_self && memo.to_utf8().is_none() {
-                            continue;
+                        // If this is change (funds to ourself) without a meaningful memo, skip it.
+                        // memo.to_utf8() returns Some(Ok("")) for empty memos (all-zero bytes),
+                        // so we must also check for empty strings, not just None.
+                        if is_to_self {
+                            let has_memo = match memo.to_utf8() {
+                                Some(Ok(ref s)) if !s.is_empty() => true,
+                                _ => false,
+                            };
+                            if !has_memo {
+                                continue;
+                            }
                         }
                         // Update the WalletTx 
                         // Do it in a short scope because of the write lock.