fix a bug (when init_emb is not None , get an error)

fastNLP/modules/encoder/embedding.py

@@ -15,7 +15,7 @@ class Embedding(nn.Module):
     def __init__(self, nums, dims, padding_idx=0, sparse=False, init_emb=None, dropout=0.0):
         super(Embedding, self).__init__()
         self.embed = nn.Embedding(nums, dims, padding_idx, sparse=sparse)
-        if init_emb:
+        if init_emb is not None:
             self.embed.weight = nn.Parameter(init_emb)
         self.dropout = nn.Dropout(dropout)