Fix bugs in Cybook/Kindle drivers

src/calibre/devices/cybookg3/driver.py

@@ -32,6 +32,7 @@ class CYBOOKG3(USBMS):
     STORAGE_CARD_VOLUME_LABEL = 'Cybook Gen 3 Storage Card'
     
     EBOOK_DIR_MAIN = "eBooks"
+    EBOOK_DIR_CARD = "eBooks"
     SUPPORTS_SUB_DIRS = True
     
     def upload_books(self, files, names, on_card=False, end_session=True, 

src/calibre/ebooks/metadata/__init__.py

@@ -21,7 +21,10 @@ def string_to_authors(raw):
     return authors
 
 def authors_to_string(authors):
+    if authors is not None:
         return ' & '.join([a.replace('&', '&&') for a in authors])
+    else:
+        return ''
 
 def author_to_author_sort(author):
     tokens = author.split()