Adjust documentation for RefreshedTemplate#getLines to unbreak CI

getLines() -- or more accurately, getMessageAsArray() -- *can* return null if the supplied $messageKey is disabled, for example.
That's why all callers of getLines() check if getLines() returned an array before iterating over its return value.

Follow-up to 62f0088a51d531a257efa4912f68b11a4a102b6e.

Change-Id: I9cd585b5ad568462f492c94e2490ea601f59b5dc