Don't attempt to create context menu for 'add printer' entry

As the 'printer' variable becomes null and the activity crashes.

This could also be solved to register context menu's for each printer
entry (and not for the 'add printer' entry). Then we would need to
create a custom view for the printer entry that returns a
ContextMenuInfo containing the printer ID. While this solution is
cleaner, it is much more complex.

Test: Tried to open context menu in 'add printer' entry
Change-Id: I23dcba2dd876cff27b5cad9736f5ec4524985bac
Fixes: 73126748