EchoPlainTextEmailFormatter: correctly handle getPrimaryLink() returning false
EchoPlainTextEmailFormatter: correctly handle getPrimaryLink() returning false

It is declared to be able to, but is not checked here expecting an array, resulting in `PHP Notice: Trying to access array offset on value of type bool` if false.

Change-Id: I8b3ebdd3b5e6fea9c84816b890198194951ea5c0

wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo