[wasm] Differentiate unnamed and empty names

src/frames.h

Index: src/frames.h
diff --git a/src/frames.h b/src/frames.h
index 4163d6f82e379dd3efe256c23d4a378f110fbe78..b66b9fdc52b16474c4c2411cb78780845c92f7e4 100644
--- a/src/frames.h
+++ b/src/frames.h
@@ -971,7 +971,9 @@ class WasmFrame : public StandardFrame {
   Object* wasm_obj();
   uint32_t function_index();
 
-  Object* function_name();
+  // The function name, or nullptr if the function is unnamed, the name is not a
+  // valid UTF-8 string, or no wasm_obj is available (in testing).
+  String* function_name();
 
   static WasmFrame* cast(StackFrame* frame) {
     DCHECK(frame->is_wasm());