Nucache NullReferenceException when copying (#7961)

(cherry picked from commit b30db05cc7)

# Conflicts:
#	src/Umbraco.Tests/PublishedContent/NuCacheChildrenTests.cs

src/Umbraco.Web/PublishedCache/NuCache/ContentStore.cs

@@ -873,9 +873,11 @@ namespace Umbraco.Web.PublishedCache.NuCache
             var id = content.FirstChildContentId;
             while (id > 0)
             {
+                // get the required link node, this ensures that both `link` and `link.Value` are not null
                 var link = GetRequiredLinkedNode(id, "child", null);
-                ClearBranchLocked(link.Value);
-                id = link.Value.NextSiblingContentId;
+                var linkValue = link.Value; // capture local since clearing in recurse can clear it
+                ClearBranchLocked(linkValue); // recurse
+                id = linkValue.NextSiblingContentId;
             }
         }