PHPstan improvements

[timkelty]

ElementQuery return type hints

Short of properly returning static, this fixes static analysis (PHPstan) for ElementQuery setters.

Without this PR, the following code causes a PHPStan error, because Entry::slug returns ElementQuery (self) and not EntryQuery (static).

Entry::find()->slug('foo')->section('foo');
// Call to an undefined method craft\elements\db\ElementQuery::section().

\craft\db\Query::one

We return null, so should be typed as such. Prevents PHPstan complaining about comparisons like:

$query->one() === null