Fix timestamp comparison failing when update API returns null

Addresses a rare instance of update checks failing when locally
installed packages point to a shared ID that has yet to be updated
to 2.1. In those instances Anki's update API returns null, which
causes a ValueError downstream when comparing the timestamps
against each other.

aqt/addons.py

@@ -325,7 +325,7 @@ Are you sure you want to continue?"""
         updated = []
         for dir, ts in mods:
             sid = str(dir)
-            if self.addonMeta(sid).get("mod",0) < ts:
+            if self.addonMeta(sid).get("mod", 0) < (ts or 0):
                 updated.append(sid)
         return updated