bug fix in preprocessor

TTS/tts/datasets/preprocess.py

@@ -35,7 +35,7 @@ def load_meta_data(datasets, eval_split=True):
             meta_data_eval_all += meta_data_eval
         meta_data_train_all += meta_data_train
         # load attention masks for duration predictor training
-        if 'meta_file_attn_mask' in dataset:
+        if 'meta_file_attn_mask' in dataset and dataset['meta_file_attn_mask'] is not None:
             meta_data = dict(load_attention_mask_meta_data(dataset['meta_file_attn_mask']))
             for idx, ins in enumerate(meta_data_train_all):
                 attn_file = meta_data[ins[1]].strip()