releasenotes: minor bug fix for index error while serializing

Differential Revision: https://phab.mercurial-scm.org/D356

hgext/releasenotes.py

@@ -426,7 +426,7 b' def serializenotes(sections, notes):'
 
             lines.append('')
 
-    if lines[-1]:
+    if lines and lines[-1]:
         lines.append('')
 
     return '\n'.join(lines)