fix: empty ZIP file

This fixes the issues raised in
#77 where a ZIP file that has no
members would raise

UnexpectedSignatureError: b'PK\x05\x06'

There was an assumption in the code that either there would be no bytes at all,
or there would be some bytes and at least one member.

(I'm not entirely sure that no bytes should be treated as a valid ZIP come to
think of it... but leaving that behaviour as it was)

test.py

@@ -379,6 +379,16 @@ def yield_input():
 
         self.assertEqual(files, [(b'first.txt', 0, b'')])
 
+    def test_empty_zip(self):
+        def yield_input():
+            file = io.BytesIO()
+            with zipfile.ZipFile(file, 'w', zipfile.ZIP_DEFLATED) as zf:
+                pass
+
+            yield file.getvalue()
+
+        self.assertEqual(list(stream_unzip(yield_input())), [])
+
     def test_not_zip(self):
         with self.assertRaises(UnexpectedSignatureError):
             next(stream_unzip([b'This is not a zip file']))