gh-98286: handle empty filename in ZipFile/ZipInfo properly

[obfusk]

• Issue: ZipFile & ZipInfo functions raise IndexError for empty filenames #98286

[obfusk]

NB: this also raises the ValueError for file names consisting entirely of slashes (which are considered directories, even if they actually contain data), which seems correct to me, but (unlike getting rid of the IndexErrors) could potentially break some existing code (though all tests are green).

>>> import zipfile
>>> fh = zipfile.ZipFile("x.zip")
>>> fh.infolist()
[<ZipInfo filename='///' filemode='drwxrwxr-x' external_attr=0x10>]
>>> fh.extract("///", "some_dir")
ValueError: Empty filename.

[obfusk]

Note that unzip fails to extract file names consisting entirely of slashes as well though:

$ unzip x.zip
Archive:  x.zip
warning:  stripped absolute path spec from ///
mapname:  conversion of  failed