refactor: add FlatStateDeltaMetadata::has_changes (#10647)

This PR is a follow-up for [this
comment](https://github.com/near/nearcore/pull/10608/files#r1499133795).

This expresses the contract around `prev_block_with_changes` with a
separate function since `prev_block_with_changes.is_none()` check is
confusing
```
    /// `None` if the block itself has flat state changes.
    /// `Some` if the block has no flat state changes, and contains
    /// info of the last block with some flat state changes.
```

core/store/src/flat/delta.rs

@@ -31,6 +31,13 @@ pub struct FlatStateDeltaMetadata {
     pub prev_block_with_changes: Option<BlockWithChangesInfo>,
 }
 
+impl FlatStateDeltaMetadata {
+    #[inline]
+    pub fn has_changes(&self) -> bool {
+        self.prev_block_with_changes.is_none()
+    }
+}
+
 #[derive(BorshSerialize, BorshDeserialize, Debug)]
 pub struct KeyForFlatStateDelta {
     pub shard_uid: ShardUId,

core/store/src/flat/storage.rs

@@ -255,10 +255,7 @@ impl FlatStorage {
         let mut deltas = HashMap::new();
         for delta_metadata in deltas_metadata {
             let block_hash = delta_metadata.block.hash;
-            let changes: CachedFlatStateChanges = if delta_metadata
-                .prev_block_with_changes
-                .is_none()
-            {
+            let changes: CachedFlatStateChanges = if delta_metadata.has_changes() {
                 store_helper::get_delta_changes(&store, shard_uid, block_hash)
                     .expect("failed to read flat state delta changes")
                     .unwrap_or_else(|| {