Set last_seen even if not blank

[aysiu]

The code as it's written now will never update last_seen unless it's blank. So it will be set once, and then never updated again. That effectively breaks the days_between_notification preference, because the span between last_seen and now will get only bigger as time goes by, and so the notifications will keep popping up every half hour.

This PR eliminates the check for whether the last_seen pref is blank, and just updates last_seen regardless.

Since right before this snippet of code, there is an exit 0 if the days_between_notifications setting is respected, last_seen will not be updated if it's been less than the number of days between notifications.

[aysiu]

In looking at the code again, I'm actually thinking it'd be better to have last_seen get set here:
https://github.com/erikng/nudge/blob/master/payload/Library/nudge/Resources/nudge#L691

That's when the actual notification pops up.

Based on my testing, this seems the best place to put it.

[aysiu]

Closing for now. Not 100% certain this fixes the issue. Have to do more testing and debugging.