Fix crash in lldb-vscode when missing function name

[zhyty]

Summary

In cases where the PC has no function name, lldb-vscode crashes.

lldb::SBFrame::GetDisplayFunctionName() returns a nullptr, and when we
attempt to construct an std::string, it raises an exception.

Test plan:
This can be observed with creating a test file (credit to @clayborg for the
example):

int main() {
  typedef void (*FooCallback)();
  FooCallback foo_callback = (FooCallback)0;
  foo_callback(); // Crash at zero!
  return 0;
}

and attempting to debug the file through VSCode.

I will add a test case in D156732 which should cover this.

[clayborg]

lldb/tools/lldb-vscode/JSONUtils.cpp

699–703

lldb/tools/lldb-vscode/JSONUtils.cpp

 //   },
 //   "required": [ "id", "name", "line", "column" ]
 // }
 llvm::json::Value CreateStackFrame(lldb::SBFrame &frame) {
   llvm::json::Object object;
   int64_t frame_id = MakeVSCodeFrameID(frame);
   object.try_emplace("id", frame_id);
 
-  std::string frame_name = frame.GetDisplayFunctionName();
+  // `function_name` can be a nullptr, which throws an error when assigned to an
+  // `std::string`.
+  const char *function_name = frame.GetDisplayFunctionName();
+  std::string frame_name =
+      function_name == nullptr ? std::string() : function_name;
   if (frame_name.empty())
     frame_name = "<unknown>";
   bool is_optimized = frame.GetFunction().GetIsOptimized();
   if (is_optimized)
     frame_name += " [opt]";
   EmplaceSafeString(object, "name", frame_name);
 
   auto source = CreateSource(frame);