parsers: relax JATS parser date handling

hepcrawl/parsers/jats.py

@@ -62,7 +62,8 @@ def parse(self):
             self.builder.add_doi(**doi)
         for keyword in self.keywords:
             self.builder.add_keyword(**keyword)
-        self.builder.add_imprint_date(self.publication_date.dumps())
+        if self.publication_date:
+            self.builder.add_imprint_date(self.publication_date.dumps())
         for reference in self.references:
             self.builder.add_reference(reference)
 
@@ -284,11 +285,10 @@ def publication_date(self):
             './front//pub-date[starts-with(@date-type,"pub")] |'
             './front//date[starts-with(@date-type,"pub")]'
         )
-        publication_date = min(
-            self.get_date(date_node) for date_node in date_nodes
-        )
-
-        return publication_date
+        if date_nodes:
+            return min(
+                self.get_date(date_node) for date_node in date_nodes
+            )
 
     @property
     def publication_info(self):
@@ -366,11 +366,10 @@ def year(self):
             not_online=not_online
         )
 
-        year = min(
-            self.get_date(date_node) for date_node in date_nodes
-        ).year
-
-        return year
+        if date_nodes:
+            return min(
+                self.get_date(date_node) for date_node in date_nodes
+            ).year
 
     def get_author_affiliations(self, author_node):
         """Extract an author's affiliations."""