Fix TokenTextEncoder constructor

Author: Matt Jones

tensor2tensor/data_generators/text_encoder.py

@@ -109,12 +109,12 @@ class TokenTextEncoder(TextEncoder):
 
   def __init__(self, vocab_filename, reverse=False, num_reserved_ids=2):
     """Initialize from a file, one token per line."""
+    super(TokenTextEncoder, self).__init__(num_reserved_ids=num_reserved_ids)
+
     self._reverse = reverse
-    if vocab_filename is None:
+    if vocab_filename is not None:
       self._load_vocab_from_file(vocab_filename)
 
-    super(TokenTextEncoder, self).__init__(num_reserved_ids=num_reserved_ids)
-
   def encode(self, sentence):
     """Converts a space-separated string of tokens to a list of ids."""
     ret = [self._token_to_id[tok] for tok in sentence.strip().split()]