ensure that a span is not skipped just because its head is 0

Author: Jemoka

stanza/utils/datasets/coref/convert_udcoref.py

@@ -163,7 +163,7 @@ def process_documents(docs, augment=False):
                         # words from 0, so we have to subtract 1 from the stanza heads
                         #print(span, candidate_head, parsed_sentence.words[candidate_head].head - 1)
                         # treat the head of the phrase as the first word that has a head outside the phrase
-                        if parsed_sentence.all_words[candidate_head].head and (
+                        if (parsed_sentence.all_words[candidate_head].head is not None) and (
                                 parsed_sentence.all_words[candidate_head].head - 1 < span[1] or
                                 parsed_sentence.all_words[candidate_head].head - 1 > span[2]
                         ):