BUG: 1.4.0 does not preserve initially empty Index and appended by `loc` assignment.

[ketozhang]

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Reproducible Example

import pandas as pd

df = pd.DataFrame(columns=["a", "b"]).set_index("a")
print(df.index, id(df.index))  # Index([], dtype='object', name='a') 4644041536
df.loc[0] = {"b": 0}
print(df.index, id(df.index))  # Int64Index([0], dtype='int64') 4726025904

Issue Description

Given an empty dataframe with a index column name set, when adding a row on a specific index using df.loc[INDEX], the original index is not preserved. Furthermore, what gets returned is not typeIndex but is Int64Index when the passed in index is an integer.

Expected Behavior

The modified index should print out Index([0], dtype='int64', name='a').

Installed Versions

1.4.0

[phofl]

@jbrockmendel This was changed in #43507, looks somehow intended. Did you mean to change this also for the index?

[jbrockmendel]

IIRC the dtype change is expected, but losing the name is not