Ignore empty sections in notes

Users may do this, and it doesn't make sense to explode when a valid
section is stated but empty.

Change-Id: I1b30be6122893563cdc4c41172e28ed5a69a0e3c
Closes-Bug: #1588992

reno/formatter.py

@@ -68,6 +68,7 @@ def format_report(loader, versions_to_include, title=None):
             notes = [
                 n
                 for fn, sha in notefiles
+                if file_contents[fn].get(section_name)
                 for n in file_contents[fn].get(section_name, [])
             ]
             if notes:

reno/tests/test_formatter.py

@@ -42,6 +42,7 @@ class TestFormatter(base.TestCase):
             'features': [
                 'We added a feature!',
             ],
+            'upgrade': None,
         },
     }