propagate dropna in get_grouper

Author: arw2019

pandas/core/groupby/grouper.py

@@ -820,7 +820,9 @@ def is_in_obj(gpr) -> bool:
         groupings.append(Grouping(Index([], dtype="int"), np.array([], dtype=np.intp)))
 
     # create the internals grouper
-    grouper = ops.BaseGrouper(group_axis, groupings, sort=sort, mutated=mutated)
+    grouper = ops.BaseGrouper(
+        group_axis, groupings, sort=sort, mutated=mutated, dropna=dropna
+    )
     return grouper, exclusions, obj