If an add-on folder contains only number, but does not contains a
meta.json file, or if this file does not contains a "mod" value, then
the following uninformative message error occur:
```Python
File "aqt/addons.py", line 387, in onCheckForUpdates
File "aqt/addons.py", line 183, in checkForUpdates
File "aqt/addons.py", line 199, in _updatedIds
<class 'TypeError'>: '<' not supported between instances of 'NoneType' and 'int'
```
This is because there is a .get in a code while the None value makes
no sens. Thus, I replaced None by a 0 value. Which ensure that, if the
last modification time is missing, the update will be done. Three case
may occur:
* either the addon is already up to date, and it's only a waste of
bandwidth
* either the add-on is not up to date, and updating was the initial
goal anyway
* Or some change did occur in the add-on folder (which is actually
probably, since it would explain the "missing mod problem"; in this
case this change may be lost, but thout would be the same problem
if the mod number was still there.
Other solutions which I could implement would be:
* asking for the user whether they want to update
* considering that it's not an ankiweb related add-on anymore, and
ignore it.
c670dd65b4