Fix pending change after flushing RegisterView (linera-io#3133)

## Motivation

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Briefly describe the goal(s) of this PR.
-->
@afck ran into an issue where an assertion caught unexpected pending
changes after voting for a block proposal. After investigating the
issue, the cause seems to be that `RegisterView::flush` had an edge case
where it wouldn't clear it's `update` field.

## Proposal

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Summarize the proposed changes and how they address the goal(s) stated
above.
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Clear the `update` field after successfully flushing.

## Test Plan

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perform as intended.

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a way that others
can reproduce the results.
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A unit test was added to reproduce the issue.

## Release Plan

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hotfixes.

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- These changes should be backported to the latest `devnet` branch, then
    - be released in a validator hotfix.
- These changes should be backported to the latest `testnet` branch,
then
    - be released in a validator hotfix.

## Links

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- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

linera-views/src/views/register_view.rs

@@ -99,6 +99,7 @@ where
             self.stored_value = value;
         }
         self.delete_storage_first = false;
+        self.update = None;
         Ok(delete_view)
     }
 

linera-views/src/views/unit_tests/views.rs

@@ -480,6 +480,22 @@ async fn test_reentrant_collection_view_has_pending_changes_after_try_load_entri
     Ok(())
 }
 
+/// Checks if a cleared [`RegisterView`] has no pending changes after flushing.
+#[tokio::test]
+async fn test_flushing_cleared_register_view() -> anyhow::Result<()> {
+    let context = create_test_memory_context();
+    let mut view = RegisterView::<_, bool>::load(context.clone()).await?;
+
+    assert!(!view.has_pending_changes().await);
+    view.clear();
+    assert!(view.has_pending_changes().await);
+
+    save_view(&context, &mut view).await?;
+    assert!(!view.has_pending_changes().await);
+
+    Ok(())
+}
+
 /// Saves a [`View`] into the [`MemoryContext<()>`] storage simulation.
 async fn save_view<C>(context: &C, view: &mut impl View<C>) -> anyhow::Result<()>
 where