fix: Article publish date is always replaced by current timestamp

[LeoniePhiline]

Fixes #281

Explanation:

In b77f22d#diff-62c740cd7ba615b9771602fedd2cab6bc50ac4052c3f8432913f59dbaf3d5effR48 an attempted migration away from the deprecated DBAL fetch() method failed.
Instead of the drop-in replacement fetchAssociative, the wrong method fetchOne was chosen.
Maybe the author @benjaminkott had intended to use fetchOne, but select the desired field rather than the entire row. (The latter approach is taken by this PR.)

In v12.0.0 (or at any time after b77f22d), the database loaded and transmitted the entire row, but only the first column of the first row was returned. (See Doctrine\DBAL\ForwardCompatibility\Result::fetchOne, which calls fetchNumeric internally.)

Note that loading and transmitting the entire database row (SELECT *) is wasteful and slow.

Therefore, instead of fixing the issue by using fetchAssociative, this PR fixes the issue by keeping fetchOne but selecting only the used field ('publish_date').

If no row is found, fetchOne will return false. A strictly typed check for false is performed before comparing (again, strictly) to 0, in which case the current time will be used.

[benjaminkott]

@LeoniePhiline thx a lot! Seems to be an oversight of me, I've added some tests to prove its working now.

[LeoniePhiline]

Yeah, quite a typical little mistake when similar changes are made in lots of places in a batch. :)

Thanks for your quick response, merge and release!