REGR: Avoid unnecessary warning when setting empty dataframe

[phofl]

• closes BUG: df.fillna(dict, inplace=True) triggers FutureWarning in 1.5.0rc0 when DataFrame is empty #48480 (Replace xxxx with the Github issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[mroeschke]

Thanks @phofl

[ryan-11055]

I receive this warning

'''
FutureWarning: In a future version, df.iloc[:, i] = newvalswill attempt to set the values inplace instead of always setting a new array. To retain the old behavior, use eitherdf[df.columns[i]] = newvalsor, if columns are non-unique,df.isetitem(i, newvals)`
'''

when calling df.update(df2). I'm unclear if this issue (#48853) fixed the warning message and / or if there is a different method I should use to update my dataframe.

[rpkilby]

@ryanlampkin are you using new-style/nullable-dtypes such as string or Int64? I've found that df.update loses the original dtype, demoting the column to object. I think this confuses some internal check in df.update, resulting in the df.loc assignment and this warning.