Return None before taking latest tag to avoid index error

bob/devtools/release.py

@@ -138,6 +138,8 @@ def get_latest_tag_name(gitpkg):
         for tag in latest_tags
         if StrictVersion.version_re.match(tag.name[1:])
     ]
+    if not tag_names: # no tags wee found.
+        return None
     # sort them correctly according to each subversion number
     tag_names.sort(key=StrictVersion)
     # take the last one, as it is the latest tag in the sorted tags