encoding: handle empty string in toutf8

mercurial/encoding.py

@@ -336,7 +336,7 b' def toutf8b(s):'
         return s._utf8
 
     try:
-        if s.decode('utf-8'):
+        s.decode('utf-8')
             return s
     except UnicodeDecodeError:
         # surrogate-encode any characters that don't round-trip