Fixed issue rendering nested empty placeholders (#6)

Currently, empty Placeholders are not rendering the same HTML that JSS
does. This PR updates the SDK's TagHelper to generate HTML that matches
the same output from JSS.

## Description / Motivation
When a Placeholder is rendered without any components loaded inside of
it, it needs to be wrapped in a div to be correctly rendered in Pages
and Experience Editor. The div being created by for this SDK is `<div
className=\"sc-empty-placeholder\"></div>`

## Testing
- [x] The Unit & Intergration tests are passing.
- [x] I have added the necesary tests to cover my changes.

## Terms
- [x] I agree to follow this project's [Code of
Conduct](CODE_OF_CONDUCT.md).

---------

Co-authored-by: Ivan Lieckens <[email protected]>

src/Sitecore.AspNetCore.SDK.RenderingEngine/TagHelpers/PlaceholderTagHelper.cs

@@ -70,8 +70,13 @@ public override async Task ProcessAsync(TagHelperContext context, TagHelperOutpu
             return;
         }
 
-        bool foundPlaceholderFeatures = false;
+        bool emptyEdit = IsInEditingMode(renderingContext) && IsPlaceHolderEmpty(placeholderFeatures);
+        if (emptyEdit)
+        {
+            output.Content.AppendHtml("<div class=\"sc-empty-placeholder\">");
+        }
 
+        bool foundPlaceholderFeatures = false;
         foreach (IPlaceholderFeature placeholderFeature in placeholderFeatures.OfType<IPlaceholderFeature>())
         {
             foundPlaceholderFeatures = true;
@@ -98,12 +103,27 @@ public override async Task ProcessAsync(TagHelperContext context, TagHelperOutpu
             output.Content.AppendHtml(html);
         }
 
+        if (emptyEdit)
+        {
+            output.Content.AppendHtml("</div>");
+        }
+
         if (!foundPlaceholderFeatures)
         {
             output.Content.SetHtmlContent($"<!-- {string.Format(Resources.Warning_PlaceholderWasEmpty, placeholderName)} -->");
         }
     }
 
+    private static bool IsInEditingMode(ISitecoreRenderingContext renderingContext)
+    {
+        return renderingContext.Response?.Content?.Sitecore?.Context?.IsEditing ?? false;
+    }
+
+    private static bool IsPlaceHolderEmpty(Placeholder placeholderFeatures)
+    {
+        return !placeholderFeatures.Exists(x => x is Component);
+    }
+
     private static Placeholder? GetPlaceholderFeatures(string placeholderName, ISitecoreRenderingContext renderingContext)
     {
         Placeholder? placeholderFeatures = null;

tests/Sitecore.AspNetCore.SDK.RenderingEngine.Tests/TagHelpers/PlaceholderTagHelperFixture.cs

@@ -476,6 +476,53 @@ public async Task ProcessAsync_PlaceholderContainsUnknownPlaceholderFeature_Outp
         tagHelperOutput.Content.GetContent().Should().BeEmpty();
     }
 
+    [Theory]
+    [AutoNSubstituteData]
+    public async Task ProcessAsync_PlaceholderContainsUnknownPlaceholderFeature_IsInEditingMode_OutputIsEditingWrapperTag(
+        PlaceholderTagHelper sut,
+        ViewContext viewContext,
+        TagHelperContext tagHelperContext,
+        TagHelperOutput tagHelperOutput)
+    {
+        // Arrange
+        SitecoreRenderingContext context = new()
+        {
+            Response = new SitecoreLayoutResponse([])
+            {
+                Content = new SitecoreLayoutResponseContent
+                {
+                    Sitecore = new SitecoreData
+                    {
+                        Context = new Context { IsEditing = true },
+                        Route = new Route
+                        {
+                            Placeholders =
+                            {
+                                [PlaceHolderWithComponentsName] =
+                                [
+                                    new TestPlaceholderFeature
+                                    {
+                                        Content = TestComponentRenderer.HtmlContent
+                                    }
+                                ]
+                            }
+                        }
+                    }
+                }
+            }
+        };
+
+        viewContext.HttpContext.SetSitecoreRenderingContext(context);
+        sut.Name = PlaceHolderWithComponentsName;
+        sut.ViewContext = viewContext;
+
+        // Act
+        await sut.ProcessAsync(tagHelperContext, tagHelperOutput);
+
+        // Assert
+        tagHelperOutput.Content.GetContent().Should().Be("<div class=\"sc-empty-placeholder\"></div>");
+    }
+
     [Theory]
     [AutoNSubstituteData]
     public async Task ProcessAsync_PlaceholderNameInLayoutServiceResponseAndPlaceholderIsNotEmpty_ContextComponentDoNotChange(