Change `ViewComponent.View()` to flow the `ViewData.Model`.

- This is more consistent with how controllers work.

#4882

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Controller.cs

@@ -138,7 +138,6 @@ namespace Microsoft.AspNetCore.Mvc
         [NonAction]
         public virtual ViewResult View(string viewName, object model)
         {
-            // Do not override ViewData.Model unless passed a non-null value.
             if (model != null)
             {
                 ViewData.Model = model;
@@ -195,7 +194,6 @@ namespace Microsoft.AspNetCore.Mvc
         [NonAction]
         public virtual PartialViewResult PartialView(string viewName, object model)
         {
-            // Do not override ViewData.Model unless passed a non-null value.
             if (model != null)
             {
                 ViewData.Model = model;

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponent.cs

@@ -262,7 +262,12 @@ namespace Microsoft.AspNetCore.Mvc
         /// <returns>A <see cref="ViewViewComponentResult"/>.</returns>
         public ViewViewComponentResult View<TModel>(string viewName, TModel model)
         {
-            var viewData = new ViewDataDictionary<TModel>(ViewData, model);
+            if (model != null)
+            {
+                ViewData.Model = model;
+            }
+
+            var viewData = new ViewDataDictionary<TModel>(ViewData);
             return new ViewViewComponentResult
             {
                 ViewEngine = ViewEngine,

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/ViewComponentTests.cs

@@ -82,6 +82,25 @@ namespace Microsoft.AspNetCore.Mvc
             Assert.Null(actualResult.ViewName);
         }
 
+        [Fact]
+        public void ViewComponent_View_WithEmptyParameter_SetsViewDataModelWithDefaultViewName()
+        {
+            // Arrange
+            var viewComponent = new TestViewComponent();
+            var model = new object();
+            viewComponent.ViewData.Model = model;
+
+            // Act
+            var actualResult = viewComponent.View();
+
+            // Assert
+            Assert.IsType<ViewViewComponentResult>(actualResult);
+            Assert.NotSame(viewComponent.ViewData, actualResult.ViewData);
+            Assert.Equal(new ViewDataDictionary<object>(viewComponent.ViewData), actualResult.ViewData);
+            Assert.Same(model, actualResult.ViewData.Model);
+            Assert.Null(actualResult.ViewName);
+        }
+
         [Fact]
         public void ViewComponent_View_WithViewNameParameter_SetsResultViewWithCustomViewName()
         {