Fix empty string column construction (#14052)

Fixes #14046 

This PR fixes empty string column construction that arises due to a corner-case in the way pyarrow constructs arrays.

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #14052

python/cudf/cudf/core/column/column.py

@@ -2438,6 +2438,21 @@ def as_column(
                     from_pandas=True if nan_as_null is None else nan_as_null,
                 )
 
+                if (
+                    isinstance(pyarrow_array, pa.NullArray)
+                    and pa_type is None
+                    and dtype is None
+                    and getattr(arbitrary, "dtype", None)
+                    == cudf.dtype("object")
+                ):
+                    # pa.array constructor returns a NullArray
+                    # for empty arrays, instead of a StringArray.
+                    # This issue is only specific to this dtype,
+                    # all other dtypes, result in their corresponding
+                    # arrow array creation.
+                    dtype = cudf.dtype("str")
+                    pyarrow_array = pyarrow_array.cast(np_to_pa_dtype(dtype))
+
                 if (
                     isinstance(arbitrary, pd.Index)
                     and arbitrary.dtype == cudf.dtype("object")

python/cudf/cudf/tests/test_dataframe.py

@@ -7256,10 +7256,7 @@ def test_dataframe_keys(df):
 def test_series_keys(ps):
     gds = cudf.from_pandas(ps)
 
-    if len(ps) == 0 and not isinstance(ps.index, pd.RangeIndex):
-        assert_eq(ps.keys().astype("float64"), gds.keys())
-    else:
-        assert_eq(ps.keys(), gds.keys())
+    assert_eq(ps.keys(), gds.keys())
 
 
 @pytest_unmark_spilling

python/cudf/cudf/tests/test_index.py

@@ -20,6 +20,7 @@
     as_index,
 )
 from cudf.testing._utils import (
+    ALL_TYPES,
     FLOAT_TYPES,
     NUMERIC_TYPES,
     OTHER_TYPES,
@@ -2703,3 +2704,26 @@ def test_index_getitem_time_duration(dtype):
                 assert gidx[i] is pidx[i]
             else:
                 assert_eq(gidx[i], pidx[i])
+
+
+@pytest.mark.parametrize("dtype", ALL_TYPES)
+def test_index_empty_from_pandas(request, dtype):
+    request.node.add_marker(
+        pytest.mark.xfail(
+            condition=not PANDAS_GE_200
+            and dtype
+            in {
+                "datetime64[ms]",
+                "datetime64[s]",
+                "datetime64[us]",
+                "timedelta64[ms]",
+                "timedelta64[s]",
+                "timedelta64[us]",
+            },
+            reason="Fixed in pandas-2.0",
+        )
+    )
+    pidx = pd.Index([], dtype=dtype)
+    gidx = cudf.from_pandas(pidx)
+
+    assert_eq(pidx, gidx)