[dask] Fix ddqdm with empty partition. (#7510)

* Fix empty partition.

* war.

python-package/xgboost/core.py

@@ -1071,7 +1071,7 @@ class DMatrix:  # pylint: disable=too-many-instance-attributes
                 c_feature_types,
                 c_bst_ulong(len(feature_types))))
 
-            if len(feature_types) != self.num_col():
+            if len(feature_types) != self.num_col() and self.num_col() != 0:
                 msg = 'feature_types must have the same length as data'
                 raise ValueError(msg)
         else:

python-package/xgboost/dask.py

@@ -1015,6 +1015,8 @@ def _maybe_dataframe(
         index = getattr(data, "index", None)
         if lazy_isinstance(data, "cudf.core.dataframe", "DataFrame"):
             import cudf
+            if prediction.size == 0:
+                return cudf.DataFrame({}, columns=columns, dtype=numpy.float32)
 
             prediction = cudf.DataFrame(
                 prediction, columns=columns, dtype=numpy.float32, index=index