Skip serializing cell ID if it's None (#6851)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Fix #6834 

## Test Plan

<!-- How was it tested? -->

Need tests?

---------

Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>

crates/ruff/resources/test/fixtures/jupyter/no_cell_id.ipynb

@@ -0,0 +1,37 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "import math\n",
+    "import os\n",
+    "\n",
+    "math.pi"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python (ruff)",
+   "language": "python",
+   "name": "ruff"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.11.3"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 2
+}

crates/ruff/src/jupyter/notebook.rs

@@ -477,8 +477,10 @@ mod tests {
     use crate::jupyter::schema::Cell;
     use crate::jupyter::Notebook;
     use crate::registry::Rule;
+    use crate::source_kind::SourceKind;
     use crate::test::{
-        read_jupyter_notebook, test_notebook_path, test_resource_path, TestedNotebook,
+        read_jupyter_notebook, test_contents, test_notebook_path, test_resource_path,
+        TestedNotebook,
     };
     use crate::{assert_messages, settings};
 
@@ -659,4 +661,22 @@ print("after empty cells")
 
         Ok(())
     }
+
+    #[test]
+    fn test_no_cell_id() -> Result<()> {
+        let path = "no_cell_id.ipynb".to_string();
+        let source_notebook = read_jupyter_notebook(path.as_ref())?;
+        let source_kind = SourceKind::Jupyter(source_notebook);
+        let (_, transformed) = test_contents(
+            &source_kind,
+            path.as_ref(),
+            &settings::Settings::for_rule(Rule::UnusedImport),
+        );
+        let linted_notebook = transformed.into_owned().expect_jupyter();
+        let mut writer = Vec::new();
+        linted_notebook.write_inner(&mut writer)?;
+        let actual = String::from_utf8(writer)?;
+        assert!(!actual.contains(r#""id":"#));
+        Ok(())
+    }
 }

crates/ruff/src/jupyter/schema.rs

@@ -150,6 +150,7 @@ pub struct CodeCell {
     /// Technically, id isn't required (it's not even present) in schema v4.0 through v4.4, but
     /// it's required in v4.5. Main issue is that pycharm creates notebooks without an id
     /// <https://youtrack.jetbrains.com/issue/PY-59438/Jupyter-notebooks-created-with-PyCharm-are-missing-the-id-field-in-cells-in-the-.ipynb-json>
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub id: Option<String>,
     /// Cell-level metadata.
     pub metadata: Value,

crates/ruff/src/test.rs

@@ -112,7 +112,7 @@ pub(crate) fn max_iterations() -> usize {
 
 /// A convenient wrapper around [`check_path`], that additionally
 /// asserts that autofixes converge after a fixed number of iterations.
-fn test_contents<'a>(
+pub(crate) fn test_contents<'a>(
     source_kind: &'a SourceKind,
     path: &Path,
     settings: &Settings,