Bug in _filelist_retrieve and extractall

[eugenkleinschmidt]

Describe the bug
This bug is present when main_streams is None. Check against None is needed.

and in extractall

Environment (please complete the following information):

• OS: Windows 10
• Python 3.8
• py7zr version: v0.5b5

Test data(please attach in the report):
test_data.zip

[miurahr]

Thank you for detailed report! Confirmed.
It is happened when there are files and directories with ALL ZERO size, then archive does not have a actual data, which main_streams holds.