[null-safety] fix real type failures in web engine (#20010)

remove extra null checks

lib/web_ui/lib/src/engine/compositor/path_metrics.dart

@@ -13,7 +13,7 @@ class CkPathMetrics extends IterableBase<ui.PathMetric>
 
   /// The [CkPath.isEmpty] case is special-cased to avoid booting the WASM machinery just to find out there are no contours.
   @override
-  Iterator<ui.PathMetric> get iterator => _path.isEmpty! ? const CkPathMetricIteratorEmpty._() : CkContourMeasureIter(_path, _forceClosed);
+  Iterator<ui.PathMetric> get iterator => _path.isEmpty ? const CkPathMetricIteratorEmpty._() : CkContourMeasureIter(_path, _forceClosed);
 }
 
 class CkContourMeasureIter implements Iterator<ui.PathMetric> {

lib/web_ui/lib/src/engine/compositor/surface.dart

@@ -160,7 +160,7 @@ class Surface {
         return _makeSoftwareCanvasSurface(htmlCanvas);
       }
 
-      return CkSurface(skSurface!, _grContext, glContext);
+      return CkSurface(skSurface, _grContext, glContext);
     }
   }