zebra-consensus: temporarily allow empty NU6.1 lockbox disbursements on Regtest

Regtest/configured testnets can be created with an empty NU6.1 lockbox disbursement list, but
block subsidy validation treated an empty list as an error on the NU6.1 activation block.

After syncing with upstream Zebra v4.2.0, this started breaking our Orchard ZSA Regtest
workflow tests.

Temporarily skip the NU6.1 lockbox disbursement check when no disbursements are configured,
and leave a FIXME to revisit whether empty configured disbursements should remain allowed or
Regtest should provide defaults.

Author: dmidem

zebra-consensus/src/block/check.rs

@@ -258,14 +258,21 @@ pub fn subsidy_is_valid(
         //
         // [ZIP-271]: <https://zips.z.cash/zip-0271>
         // [ZIP-1016]: <https://zips.z.cash/zip-101>
-        if Some(height) == NetworkUpgrade::Nu6_1.activation_height(net) {
+        if Some(height) == NetworkUpgrade::Nu6_1.activation_height(net)
+            && !net.lockbox_disbursements(height).is_empty()
+        {
             let lockbox_disbursements = net.lockbox_disbursements(height);
 
-            if lockbox_disbursements.is_empty() {
-                Err(BlockError::Other(
-                    "missing lockbox disbursements for NU6.1 activation block".to_string(),
-                ))?;
-            }
+            // FIXME: Temporarily disabled this error by commenting out the code below
+            // and adding the extra condition to the `if` above, because after syncing
+            // with upstream Zebra v4.2.0 our Orchard ZSA Regtest workflow tests
+            // started failing here when NU6.1 lockbox disbursements are empty on
+            // Regtest/configured testnets. Revisit later.
+            // if lockbox_disbursements.is_empty() {
+            //     Err(BlockError::Other(
+            //         "missing lockbox disbursements for NU6.1 activation block".to_string(),
+            //     ))?;
+            // }
 
             deferred_pool_balance_change = lockbox_disbursements.into_iter().try_fold(
                 deferred_pool_balance_change,