[7.x] Fix rendering plain text only notifications

[pawelmysior]

The ability to send plain text versions of notifications alongside html version was introduced in #33725. In #33781 I've made small fixes that allow to send plain text only notifications.

When the notification is sent, the view is built as an associative array using the buildView() method of the MailChannel class. The array returned by this method is passed to the Mailer class, which parses this view array in the parseView() method.

However, when the notifications is rendered (for previewing in the browser), the view is built as a numeric array in the render() method. When it is passed to the Mailer class, it is parsed differently in the parseView() method. If the notifications has the html view set, it is not a problem, the result is the same. But it the html view is null, the result of parseView() is different.

This PR aims to fix this inconsistency, by building the view as an associative array in the MailMessage::render() method.

[driesvints]

Should we add a test for this?

[driesvints]

Sent in #33816 to revert this.