BUG: When upsampling an empty dataframe, the index type changes from DatetimeIndex to MultiIndex

[creid-hartree]

Pandas version checks

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Reproducible Example

import pandas as pd
df = pd.DataFrame([], index=pd.DatetimeIndex([]))

# this returns a MultiIndex rather than a DatetimeIndex
df.resample("D").ffill().index

Issue Description

When upsampling an empty dataframe, the index type changes from DatetimeIndex to MultiIndex.

This applies to all the upsampling methods, i.e. resample followed by fillna, ffill, bfill, nearest, asfreq, or interpolate.

This only happens on empty dataframes or series, and only when upsampling (for example resample("D").mean() works as expected).

This regression was introduced in pandas 2.0.0

Expected Behavior

The resampled dataframe should have a DatetimeIndex rather than a MultiIndex.

Installed Versions

pandas : 2.1.1
numpy : 1.26.0
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 68.2.2
pip : 23.3
tzdata : 2023.3

all others None

[creid-hartree]

Perhaps related to these two (fixed) issues:
#47705
#53664

[Charlie-XIAO]

take