[6.3] Update public RenderIndex.node initializer to accept an optional (#1375)

Since RenderIndex.Node’s type property is optional, the public initializer should accept an optional.

Co-authored-by: Pat Shaughnessy <pat_shaughnessy@apple.com>

Author: mayaepps

Sources/SwiftDocC/Indexing/RenderIndexJSON/RenderIndex.swift

@@ -223,7 +223,7 @@ extension RenderIndex {
         public init(
             title: String,
             path: String?,
-            type: String,
+            type: String?,
             children: [Node]?,
             isDeprecated: Bool,
             isExternal: Bool,