We were getting an exception in this case, which was OK I guess since
this shouldn't happen if the application is used in a normal way, but we
can simplify the code a little bit if we make the `recipients` code
return an empty list of users.
Note that the behavior of the `AdminNotification#list_of_recipients` and
`Newsletter#list_of_recipient_emails` methods is now slightly different;
previously they returned `nil` when given an invalid segment recipient,
while now they return an empty array. I haven't found a place where this
change is relevant. For example, in both of these models, the `deliver`
method used to raise an exception when given an invalid segment while
now it doesn't, but we always check the user segment is valid before
calling the `deliver` method anyway, so it doesn't really affect the
application.
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