fix: undefined values should become null in nullable fields (#2658)

### Bug Fix: Undefined Values in Nullable Fields

**Issue**: When inserting data with `undefined` values into nullable
fields, LanceDB was incorrectly coercing them to default values (`false`
for booleans, `NaN` for numbers, `""` for strings) instead of `null`.

**Fix**: Modified the `makeVector()` function in `arrow.ts` to properly
convert `undefined` values to `null` for nullable fields before passing
data to Apache Arrow.

fixes: #2645

**Result**: Now `{ text: undefined, number: undefined, bool: undefined
}` correctly becomes `{ text: null, number: null, bool: null }` when
fields are marked as nullable in the schema.

**Files Changed**: 
- `nodejs/lancedb/arrow.ts` (core fix)
- `nodejs/__test__/arrow.test.ts` (test coverage)

- This ensures proper null handling for nullable fields as expected by
users.

---------

Co-authored-by: Will Jones <willjones127@gmail.com>

nodejs/__test__/arrow.test.ts

@@ -1039,3 +1039,33 @@ describe.each([arrow15, arrow16, arrow17, arrow18])(
     });
   },
 );
+
+// Test for the undefined values bug fix
+describe("undefined values handling", () => {
+  it("should handle mixed undefined and actual values", () => {
+    const schema = new Schema([
+      new Field("text", new Utf8(), true), // nullable
+      new Field("number", new Int32(), true), // nullable
+      new Field("bool", new Bool(), true), // nullable
+    ]);
+
+    const data = [
+      { text: undefined, number: 42, bool: true },
+      { text: "hello", number: undefined, bool: false },
+      { text: "world", number: 123, bool: undefined },
+    ];
+    const table = makeArrowTable(data, { schema });
+
+    const result = table.toArray();
+    expect(result).toHaveLength(3);
+    expect(result[0].text).toBe(null);
+    expect(result[0].number).toBe(42);
+    expect(result[0].bool).toBe(true);
+    expect(result[1].text).toBe("hello");
+    expect(result[1].number).toBe(null);
+    expect(result[1].bool).toBe(false);
+    expect(result[2].text).toBe("world");
+    expect(result[2].number).toBe(123);
+    expect(result[2].bool).toBe(null);
+  });
+});