Avoid Column materialization in RangeIndex.nans_to_nulls (#20331)

Follow up to #20314

Realized after review we can avoid materializing the `range` to a `Column`

Authors:
  - Matthew Roeschke (https://github.com/mroeschke)

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

URL: #20331

Author: mroeschke

python/cudf/cudf/core/frame.py

@@ -182,7 +182,7 @@ def deserialize(cls, header, frames):
         return cls._from_data(col_accessor)
 
     @_performance_tracking
-    def nans_to_nulls(self):
+    def nans_to_nulls(self) -> Self:
         result = []
         for col in self._columns:
             converted = col.nans_to_nulls()

python/cudf/cudf/core/index.py

@@ -3413,6 +3413,10 @@ def isin(self, values, level=None) -> cupy.ndarray:
 
         return self._column.isin(values).values
 
+    @_performance_tracking
+    def nans_to_nulls(self) -> Self:
+        return self.copy()
+
     def __pos__(self) -> Self:
         return self.copy()
 

python/cudf/cudf/core/indexed_frame.py

@@ -1890,7 +1890,7 @@ def ewm(
         )
 
     @_performance_tracking
-    def nans_to_nulls(self):
+    def nans_to_nulls(self) -> Self:
         """
         Convert nans (if any) to nulls