setitem on empty DataFrame / Series casts to RangeIndex

[TomAugspurger]

I thought we had an issue for this, but couldn't find it.

In [41]: df = pd.DataFrame(columns=['A'])

In [42]: df.index.dtype
Out[42]: dtype('O')

In [43]: df['A'] = []

In [44]: df.index.dtype
Out[44]: dtype('int64')

In [45]: df.index
Out[45]: RangeIndex(start=0, stop=0, step=1)

I this should still be empty an empty Index (though see #16961 which proposes change pd.Series() to have that use a range index. Not sure if that's a good idea or not).

xref dask/dask#2564

[TomAugspurger]

It's possible that adding an and len(value) to the if statement in

pandas/pandas/core/frame.py

Lines 3386 to 3389 in ee6b131

	# GH5632, make sure that we are a Series convertible
	if not len(self.index) and is_list_like(value):
	try:
	value = Series(value)

will fix things. That plus a test would make a good first PR.

[SvoONs]

I would like to fix this as my first PR!