Fix: Python: Empty list of drawing.strokes doesn't have length of 0

The API would return `None` if the number of strokes in the drawing was `0`.
Instead this should return a slice that has a length of 0 to be consistent.

scripts/modules/bpy_types.py

@@ -1421,5 +1421,4 @@ class GreasePencilDrawing(_StructRNA):
         """
         from _bpy_internal.grease_pencil.stroke import GreasePencilStrokeSlice
         num_strokes = self.attributes.domain_size('CURVE')
-        if num_strokes > 0:
-            return GreasePencilStrokeSlice(self, 0, num_strokes)
+        return GreasePencilStrokeSlice(self, 0, num_strokes)