fix nlp.evaluate (#4924) (#4925)

* new file:   test_issue4924.py

* modified:   spacy/gold.pyx

* modified:   test_issue4924.py for python2

spacy/gold.pyx

@@ -694,6 +694,11 @@ cdef class GoldParse:
         self.cats = {} if cats is None else dict(cats)
         self.links = links
 
+        # orig_annot is used as an iterator in `nlp.evalate` even if self.length == 0,
+        # so set a empty list to avoid error.
+        # if self.lenght > 0, this is modified latter.
+        self.orig_annot = []
+
         # avoid allocating memory if the doc does not contain any tokens
         if self.length > 0:
             if words is None:

spacy/tests/regression/test_issue4924.py

@@ -0,0 +1,16 @@
+# coding: utf8
+from __future__ import unicode_literals
+
+import pytest
+
+import spacy
+
+
+@pytest.fixture
+def nlp():
+    return spacy.blank("en")
+
+
+def test_evaluate(nlp):
+    docs_golds = [("", {})]
+    nlp.evaluate(docs_golds)