Fix regression in output padding

Closes zcash/sapling-crypto#121.

CHANGELOG.md

@@ -7,6 +7,12 @@ and this library adheres to Rust's notion of
 
 ## [Unreleased]
 
+### Fixed
+- `sapling_crypto::builder::BundleType::num_outputs` now matches the previous
+  behaviour for Sapling bundle padding, by including dummy outputs if there are
+  no requested outputs but some requested spends, and `bundle_required` is set
+  to `false` (as in `BundleType::DEFAULT`).
+
 ## [0.1.0] - 2024-01-26
 The crate has been completely rewritten. See [`zcash/librustzcash`] for the
 history of this rewrite.

src/builder.rs

@@ -90,13 +90,13 @@ impl BundleType {
         requested_outputs: usize,
     ) -> Result<usize, &'static str> {
         match self {
-            BundleType::Transactional { bundle_required } => {
-                Ok(if *bundle_required || requested_outputs > 0 {
+            BundleType::Transactional { bundle_required } => Ok(
+                if *bundle_required || requested_spends > 0 || requested_outputs > 0 {
                     core::cmp::max(requested_outputs, MIN_SHIELDED_OUTPUTS)
                 } else {
                     0
-                })
-            }
+                },
+            ),
             BundleType::Coinbase => {
                 if requested_spends == 0 {
                     Ok(requested_outputs)