SeriesGroupBy.transform cannot handle empty series #26208
Description
Code Sample, a copy-pastable example if possible
d = pd.DataFrame({1: [], 2: []})
g = d.groupby(1)
g[2].transform(lambda x: x)
Traceback (most recent call last):
File "<input>", line 1, in <module>
File "C:\python37\lib\site-packages\pandas\core\groupby\generic.py", line 945, in transform
result = concat(results).sort_index()
File "C:\python37\lib\site-packages\pandas\core\reshape\concat.py", line 228, in concat
copy=copy, sort=sort)
File "C:\python37\lib\site-packages\pandas\core\reshape\concat.py", line 262, in __init__
raise ValueError('No objects to concatenate')Problem description
Crashes on SeriesGroupby obj with zero length, which came from an empty dataframe. Would be nicer if pandas can handle this case without raising errors, by for example, just return an empty series. Thanks.
Expected Output
Series([], Name: 2, dtype: float64)