Don't read upgrade progress file if size hasn't changed

apparently that is possible sometimes?  or perhaps just an issue on python 3?

tailbone/views/upgrades.py

@@ -343,6 +343,7 @@ class UpgradeView(MasterView):
         offset = session.get('stdout.offset', 0)
         if os.path.exists(path):
             size = os.path.getsize(path) - offset
+            if size:
                 with open(path, 'rb') as f:
                     f.seek(offset)
                     chunk = f.read(size)