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

@@ -111,6 +111,9 @@ 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)
+  if not hasattr(cls, '_FIELDS'):
+    # Edge case with no fields e.g. module = nn.Module() causes error later.
+    return object.__repr__(module)
   cls_name = cls.__name__
   rep = ''