Fix reading metadata from filenames when the author regexp does not match anything

src/calibre/ebooks/metadata/__init__.py

@@ -22,6 +22,8 @@ except:
     _author_pat = re.compile(r'(?i),?\s+(and|with)\s+')
 
 def string_to_authors(raw):
+    if not raw:
+        return []
     raw = raw.replace('&&', u'\uffff')
     raw = _author_pat.sub('&', raw)
     authors = [a.strip().replace(u'\uffff', '&') for a in raw.split('&')]

src/calibre/ebooks/metadata/meta.py

@@ -149,6 +149,7 @@ def metadata_from_filename(name, pat=None):
         try:
             au = match.group('author')
             aus = string_to_authors(au)
+            if aus:
                 mi.authors = aus
                 if prefs['swap_author_names'] and mi.authors:
                     def swap(a):