fix: Add empty row in MPT table to prevent testing panics

As pointed by @lispc in
#1024 (comment),
Halo2 currently has a bug which prevents it to render correctly the
errors for an empty region with a `ConstraintError` on it.

An issue has been filled for this so that we can fix it in
privacy-scaling-explorations/halo2#117.

Meanwhile a new tag is not released for Halo2 with a fix, this fix will
temporarily be needed and a reminder to remove it has been created in #1032

zkevm-circuits/src/table.rs

@@ -534,8 +534,9 @@ impl MptTable {
         updates: &MptUpdates,
         randomness: Value<F>,
     ) -> Result<(), Error> {
+        self.assign(region, 0, &MptUpdateRow([Value::known(F::zero()); 7]))?;
         for (offset, row) in updates.table_assignments(randomness).iter().enumerate() {
-            self.assign(region, offset, row)?;
+            self.assign(region, offset + 1, row)?;
         }
         Ok(())
     }

zkevm-circuits/src/witness/mpt.rs

@@ -37,7 +37,7 @@ pub struct MptUpdates(HashMap<Key, MptUpdate>);
 
 /// The field element encoding of an MPT update, which is used by the MptTable
 #[derive(Debug, Clone, Copy)]
-pub struct MptUpdateRow<F>([F; 7]);
+pub struct MptUpdateRow<F>(pub(crate) [F; 7]);
 
 impl MptUpdates {
     pub(crate) fn get(&self, row: &Rw) -> Option<MptUpdate> {