* Fix issue #19: Allow parsing/pos tagging of empty strings

spacy/en/pos.pyx

@@ -265,6 +265,8 @@ cdef class EnPosTagger:
         Args:
             tokens (Tokens): The tokens to be tagged.
         """
+        if tokens.length == 0:
+            return 0
         cdef int i
         cdef atom_t[N_CONTEXT_FIELDS] context
         cdef const weight_t* scores

spacy/syntax/parser.pyx

@@ -72,6 +72,9 @@ cdef class GreedyParser:
             Transition guess
             uint64_t state_key
 
+        if tokens.length == 0:
+            return 0
+
         cdef atom_t[CONTEXT_SIZE] context
         cdef int n_feats
         cdef Pool mem = Pool()