Fix groupby head/tail for empty dataframe

[shwina]

Description

Closes #13397

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[wence-]

The same error occurs (previously and after the fix for scans in #13389, which was me again sorry!). Although it's impossible to reach that code path right now because mimic pandas order for scans only happens if the dataframe is not empty.

But can you apply this patch too please?

diff --git a/python/cudf/cudf/core/groupby/groupby.py b/python/cudf/cudf/core/groupby/groupby.py
index fb242a49ad..b3be6d9de0 100644
--- a/python/cudf/cudf/core/groupby/groupby.py
+++ b/python/cudf/cudf/core/groupby/groupby.py
@@ -2277,9 +2277,7 @@ class GroupBy(Serializable, Reducible, Scannable):
         # result coming back from libcudf has null_count few rows than
         # the input, so we must produce an ordering from the full
         # input range.
-        _, (ordering,), _ = self._groupby.groups(
-            [arange(0, self.obj._data.nrows)]
-        )
+        _, (ordering,), _ = self._groupby.groups([arange(0, len(self.obj))])
         if self._dropna and any(
             c.has_nulls(include_nan=True) > 0
             for c in self.grouping._key_columns
@@ -2287,7 +2285,7 @@ class GroupBy(Serializable, Reducible, Scannable):
             # Scan aggregations with null/nan keys put nulls in the
             # corresponding output rows in pandas, to do that here
             # expand the result by reindexing.
-            ri = cudf.RangeIndex(0, self.obj._data.nrows)
+            ri = cudf.RangeIndex(0, len(self.obj))
             result.index = cudf.Index(ordering)
             # This reorders and expands
             result = result.reindex(ri)

[shwina]

Done

[shwina]

/merge