feat(chain, file_store): add is_empty method to PersistBackend trait

crates/chain/src/persist.rs

@@ -80,6 +80,15 @@ pub trait PersistBackend<C> {
 
     /// Return the aggregate changeset `C` from persistence.
     fn load_from_persistence(&mut self) -> Result<C, Self::LoadError>;
+
+    /// Returns whether the persistence backend contains no data.
+    fn is_empty(&mut self) -> Result<bool, Self::LoadError>
+    where
+        C: Append,
+    {
+        self.load_from_persistence()
+            .map(|changeset| changeset.is_empty())
+    }
 }
 
 impl<C: Default> PersistBackend<C> for () {
@@ -94,4 +103,8 @@ impl<C: Default> PersistBackend<C> for () {
     fn load_from_persistence(&mut self) -> Result<C, Self::LoadError> {
         Ok(C::default())
     }
+
+    fn is_empty(&mut self) -> Result<bool, Self::LoadError> {
+        Ok(true)
+    }
 }