Follow up PR: pandas-dev#28097 Simplify branch statement

Author: proost

pandas/core/indexes/multi.py

@@ -1308,10 +1308,7 @@ def _get_grouper_for_level(self, mapper, level):
             # Remove unobserved levels from level_index
             level_index = level_index.take(uniques)
 
-        if len(level_index):
-            grouper = level_index.take(codes)
-        else:
-            grouper = level_index.take(codes, fill_value=True)
+        grouper = level_index.take(codes, fill_value=True)
 
         return grouper, codes, level_index