Fix error when printing module with no fields.

---

Currently, the edge case code will cause an error:

```
print(nn.Module())
```

PiperOrigin-RevId: 678109450

flax/linen/module.py

@@ -111,12 +111,17 @@ def _attr_repr(value: Any):
 def _module_repr(module: 'Module', num_spaces: int = 4):
   """Returns a pretty printed representation of the module."""
   cls = type(module)
+  try:
+    fields = dataclasses.fields(cls)
+  except TypeError:
+    # Edge case with no fields e.g. module = nn.Module() causes error later.
+    return object.__repr__(module)
   cls_name = cls.__name__
   rep = ''
 
   attributes = {
     f.name: f.type
-    for f in dataclasses.fields(cls)
+    for f in fields
     if f.name not in ('parent', 'name') and f.repr
   }
   child_modules = {