Do not emit topics section for variants that don't have topics

If the Objective-C version of a symbol doesn't have topics, do not emit
a Topics section for it, instead of defaulting to the Swift symbol's
topics. This logic was implemented for Topics sections in articles, but
not in symbols.

rdar://92127060

Author: franklinsch

Sources/SwiftDocC/Model/Rendering/RenderNodeTranslator.swift

@@ -1279,11 +1279,7 @@ public struct RenderNodeTranslator: SemanticVisitor {
                 )
             }
 
-            if sections.isEmpty {
-                return nil
-            } else {
-                return sections
-            }
+            return sections
         } ?? .init(defaultValue: [])
 
         node.defaultImplementationsSectionsVariants = VariantCollection<[TaskGroupRenderSection]>(

Tests/SwiftDocCTests/Rendering/RenderNodeTranslatorSymbolVariantsTests.swift

@@ -701,7 +701,7 @@ class RenderNodeTranslatorSymbolVariantsTests: XCTestCase {
         )
     }
 
-    func testEncodesNilTopicsSectionsForVariantIfDefaultIsNonEmpty() throws {
+    func testEncodesNilTopicsSectionsForArticleVariantIfDefaultIsNonEmpty() throws {
         try assertMultiVariantArticle(
             configureArticle: { article in
                 article.automaticTaskGroups = []
@@ -738,6 +738,26 @@ class RenderNodeTranslatorSymbolVariantsTests: XCTestCase {
         )
     }
 
+    func testEncodesNilTopicsSectionsForSymbolVariantIfDefaultIsNonEmpty() throws {
+        try assertMultiVariantSymbol(
+            assertOriginalRenderNode: { renderNode in
+                XCTAssertEqual(renderNode.topicSections.count, 6)
+            },
+            assertDataAfterApplyingVariant: { renderNodeData in
+                // See reasoning for the RenderNodeProxy type in the similar test above.
+                
+                struct RenderNodeProxy: Codable {
+                    var topicSections: [TaskGroupRenderSection]?
+                }
+                
+                XCTAssertNil(
+                    try JSONDecoder().decode(RenderNodeProxy.self, from: renderNodeData).topicSections,
+                    "Expected topicSections to be null in the JSON because the article has no Objective-C topics."
+                )
+            }
+        )
+    }
+    
     func testArticleAutomaticTaskGroupsForArticleOnlyIncludeTopicsAvailableInTheArticleLanguage() throws {
         func referenceWithPath(_ path: String) -> ResolvedTopicReference {
             ResolvedTopicReference(
@@ -848,7 +868,13 @@ class RenderNodeTranslatorSymbolVariantsTests: XCTestCase {
                 XCTAssertEqual(renderNode.topicSections.count, 2)
             },
             assertAfterApplyingVariant: { renderNode in
-                XCTAssertEqual(renderNode.topicSections.count, 2)
+                XCTAssert(
+                    renderNode.topicSections.isEmpty,
+                    """
+                    Expected no topics section for the Objective-C variant, because there are no Objective-C \
+                    relationships.
+                    """
+                )
             }
         )
     }
@@ -1040,7 +1066,8 @@ class RenderNodeTranslatorSymbolVariantsTests: XCTestCase {
         configureSymbol: (Symbol) throws -> () = { _ in },
         configureRenderNodeTranslator: (inout RenderNodeTranslator) -> () = { _ in },
         assertOriginalRenderNode: (RenderNode) throws -> (),
-        assertAfterApplyingVariant: (RenderNode) throws -> ()
+        assertAfterApplyingVariant: (RenderNode) throws -> () = { _ in },
+        assertDataAfterApplyingVariant: (Data) throws -> () = { _ in }
     ) throws {
         let (_, bundle, context) = try testBundleAndContext(copying: "TestBundle")
 
@@ -1066,7 +1093,8 @@ class RenderNodeTranslatorSymbolVariantsTests: XCTestCase {
             identifier: identifier,
             configureRenderNodeTranslator: configureRenderNodeTranslator,
             assertOriginalRenderNode: assertOriginalRenderNode,
-            assertAfterApplyingVariant: assertAfterApplyingVariant
+            assertAfterApplyingVariant: assertAfterApplyingVariant,
+            assertDataAfterApplyingVariant: assertDataAfterApplyingVariant
         )
     }