BUG: dataframe returned for alternative agg method call np.size and "size" are inconsistent
- [ ] closes #47959
- [ ] [Tests added ]

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