Revert feat(normalization): add validation for empty features in NormalizerProcessorStep and UnnormalizerProcessorStep (#2087)

Revert "feat(normalization): add validation for empty features in NormalizerProcessorStep and UnnormalizerProcessorStep (#2087)"

This reverts commit f173265354.

src/lerobot/processor/normalize_processor.py

@@ -108,18 +108,16 @@ class _NormalizationMixin:
         """
         # Track if stats were explicitly provided (not None and not empty)
         self._stats_explicitly_provided = self.stats is not None and bool(self.stats)
-        # Check if self.features is not empty
-        if not self.features:
-            raise ValueError("Normalization features cannot be empty")
         # Robust JSON deserialization handling (guard empty maps).
-        first_val = next(iter(self.features.values()))
-        if isinstance(first_val, dict):
-            reconstructed = {}
-            for key, ft_dict in self.features.items():
-                reconstructed[key] = PolicyFeature(
-                    type=FeatureType(ft_dict["type"]), shape=tuple(ft_dict["shape"])
-                )
-            self.features = reconstructed
+        if self.features:
+            first_val = next(iter(self.features.values()))
+            if isinstance(first_val, dict):
+                reconstructed = {}
+                for key, ft_dict in self.features.items():
+                    reconstructed[key] = PolicyFeature(
+                        type=FeatureType(ft_dict["type"]), shape=tuple(ft_dict["shape"])
+                    )
+                self.features = reconstructed
 
         # if keys are strings (JSON), rebuild enum map
         if self.norm_map and all(isinstance(k, str) for k in self.norm_map):