BUG: dataframe returned for alternative agg method call np.size and "size" are inconsistent

[ntachukwu]

• closes API: Specify the behaviour for operating on empty objects #47959
• [Tests added ]

[ntachukwu]

import pandas as pd
import numpy as np

df = pd.DataFrame(columns=['a', 'b'])

# These return equal objects
df.agg({'a':['mean','std']})
df.agg({'a':[np.mean,np.std]})

# This does not
df.agg({'a': ['size']})
df.agg({'a': [np.size]})

[ntachukwu]

This isn't peculiar to empty inputs. And is a duplicate of #42203. I will be closing this draft PR.