BUG: make construction of labels can handle empty periods(pandas-dev#28479)

Author: andrew-buzzni

pandas/core/groupby/generic.py

@@ -9,6 +9,7 @@
 import copy
 import functools
 from functools import partial
+import itertools
 from textwrap import dedent
 import typing
 from typing import Any, Callable, FrozenSet, Iterator, Sequence, Type, Union
@@ -1264,9 +1265,12 @@ def value_counts(
 
         # num. of times each group should be repeated
         rep = partial(np.repeat, repeats=np.add.reduceat(inc, idx))
-
-        # multi-index components
-        labels = list(map(rep, self.grouper.recons_labels)) + [llab(lab, inc)]
+        
+        #multi-index components
+        try:
+            labels = list(map(rep, self.grouper.recons_labels )) + [llab(lab, inc)]
+        except ValueError:
+            labels = list(map(rep, [[k for k, _ in itertools.groupby(ids)]])) + [llab(lab, inc)]
         levels = [ping.group_index for ping in self.grouper.groupings] + [lev]
         names = self.grouper.names + [self._selection_name]