fallback to regex year extraction or empty String when arrow fails

scholarly/publication_parser.py

@@ -316,7 +316,8 @@ def fill(self, publication: Publication)->Publication:
                         publication['bib']['pub_year'] = arrow.get(val.text, patterns).year
                     except ValueError:
                         # fallback to regex year extraction if arrow fails
-                         publication['bib']['pub_year'] = re.search(r'\d{4}', val.text).group()
+                        match = re.search(r'\d{4}', val.text)
+                        publication['bib']['pub_year'] = match.group() if match else ""
                     publication['bib']['pub_date'] = val.text
                 elif key == 'description':
                     # try to find all the gsh_csp if they exist