Make pipeline work on empty docs

spacy/pipeline.pyx

@@ -482,7 +482,7 @@ class Tagger(Pipe):
                     if lemma != 0 and lemma != doc.c[j].lex.orth:
                         doc.c[j].lemma = lemma
                 idx += 1
-            if tensors is not None:
+            if tensors is not None and len(tensors):
                 if isinstance(doc.tensor, numpy.ndarray) \
                 and not isinstance(tensors[i], numpy.ndarray):
                     doc.extend_tensor(tensors[i].get())

spacy/syntax/nn_parser.pyx

@@ -217,6 +217,8 @@ cdef class Parser:
     def predict(self, docs, beam_width=1, beam_density=0.0, drop=0.):
         if isinstance(docs, Doc):
             docs = [docs]
+        if not any(len(doc) for doc in docs):
+            return self.moves.init_batch(docs)
         if beam_width < 2:
             return self.greedy_parse(docs, drop=drop)
         else: