Type of changes

• Bug fix

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @PyFilesystem/maintainers may be pedantic in the code review.

Description

This addresses an issues where write_zip() assumed that the details namespace was always available. The stat namespace will be checked first, followed by details, and finally the current time will be used if st_mtime is still None.

I'm willing to author a test against this, but I'm only aware of this being an issue with the s3 filesystem (and only when it encounters a directory) and I'm not certain how best to test that here.

#556