BUG: DataFrame.resample is changing the index type to MultiIndex when the dataframe is empty
BUG: DataFrame.resample is changing the index type to MultiIndex when the dataframe is empty

very easy bug to reproduce. the bug has been opened for 1 year.

```
# repro
import pandas as pd

df = pd.DataFrame(columns = ["a", "b"], index = pd.DatetimeIndex([]))
sampled = df.resample("8h").apply(pd.Series.mean)

print(df.index)
print(sampled.index)
```

output on pandas 1
```
DatetimeIndex([], dtype='datetime64[ns]', freq=None)
DatetimeIndex([], dtype='datetime64[ns]', freq='8H')
```

output on pandas 2 and main branch
```
DatetimeIndex([], dtype='datetime64[ns]', freq=None)
MultiIndex([], )
```


- [X] closes #55572 (Replace xxxx with the GitHub issue number)
- [X] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [X] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [X] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [X] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @morotti 