Fix error when printing module with no fields.

---

Currently, the edge case code will cause an error:

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

PiperOrigin-RevId: 679456418

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 = {