fix(echarts): Display NULL values in categorical x-axis for bar charts (#34761)

Co-authored-by: Claude <noreply@anthropic.com>

superset-frontend/plugins/plugin-chart-echarts/test/utils/series.test.ts

@@ -493,6 +493,43 @@ describe('extractSeries', () => {
     ]);
   });
 
+  it('should convert NULL x-values to NULL_STRING for categorical axis', () => {
+    const data = [
+      {
+        browser: 'Firefox',
+        count: 5,
+      },
+      {
+        browser: null,
+        count: 10,
+      },
+      {
+        browser: 'Chrome',
+        count: 8,
+      },
+    ];
+    expect(
+      extractSeries(data, {
+        xAxis: 'browser',
+        xAxisType: AxisType.Category,
+      }),
+    ).toEqual([
+      [
+        {
+          id: 'count',
+          name: 'count',
+          data: [
+            ['Firefox', 5],
+            [NULL_STRING, 10],
+            ['Chrome', 8],
+          ],
+        },
+      ],
+      [],
+      5,
+    ]);
+  });
+
   it('should do missing value imputation', () => {
     const data = [
       {