[backport] Handle missing values in dataframe with category dtype. (#7331) (#7413)

* Handle missing values in dataframe with category dtype. (#7331)

* Replace -1 in pandas initializer.
* Unify `IsValid` functor.
* Mimic pandas data handling in cuDF glue code.
* Check invalid categories.
* Fix DDM sketching.

* Fix pick error.

tests/python/test_with_pandas.py

@@ -138,9 +138,22 @@ class TestPandas:
             X, enable_categorical=True
         )
 
-        assert np.issubdtype(transformed[:, 0].dtype, np.integer)
         assert transformed[:, 0].min() == 0
 
+        # test missing value
+        X = pd.DataFrame({"f0": ["a", "b", np.NaN]})
+        X["f0"] = X["f0"].astype("category")
+        arr, _, _ = xgb.data._transform_pandas_df(X, enable_categorical=True)
+        assert not np.any(arr == -1.0)
+
+        X = X["f0"]
+        with pytest.raises(ValueError):
+            xgb.DMatrix(X, y)
+
+        Xy = xgb.DMatrix(X, y, enable_categorical=True)
+        assert Xy.num_row() == 3
+        assert Xy.num_col() == 1
+
     def test_pandas_sparse(self):
         import pandas as pd
         rows = 100