fix(charts): Time grain is None when dataset uses Jinja (#25842)

superset-frontend/plugins/plugin-chart-echarts/test/MixedTimeseries/buildQuery.test.ts

@@ -307,6 +307,7 @@ test('should convert a queryObject with x-axis although FF is disabled', () => {
     extras: {
       having: '',
       where: "(foo in ('a', 'b'))",
+      time_grain_sqla: 'P1W',
     },
     applied_time_extras: {},
     columns: [