fix(dataset_info): handle None attrs in DatasetInfo validator (#1680)

attrs is nullable in some DB configurations (e.g. Django JSONField
with null=True). Add a field_validator to coerce None to [] before
Pydantic validates the list type, consistent with existing validators
for params and metrics.

Author: amritghimire

src/datachain/lib/dataset_info.py

@@ -84,6 +84,13 @@ def validate_location(cls, v):
     def validate_metrics(cls, v):
         return cls._validate_dict(v)
 
+    @field_validator("attrs", mode="before")
+    @classmethod
+    def validate_attrs(cls, v):
+        if v is None:
+            return []
+        return v
+
     @classmethod
     def from_models(
         cls,