jobqueue: Change internal $params logic to teach Phan (3)

jobqueue: Change internal $params logic to teach Phan (3)

Follows-up 9b4938c40d02c, 4bff6f1558, and 766549999c.

It seems despite being optional and documented as 'array|Title',
Phan still thinks it is wrong to pass Title.

Try removing the explicit default of empty array,
the lower code already does this anyway.

This should fix build failures that are preventing merges
in repos where Phan is finding Job::__construct(string, Title)
where it currently fails as follows:

 CompileArticleMetadataJob.php:11
 PhanTypeMismatchArgument Argument 2 (params) is …\Title|string but
 \Job::__construct() takes array

Change-Id: I1c3aee273e0917e30b9c18b49b24fc7f966ff57c