Handle `<pre>`whitespace formatting edge case (no new line)

[dummdidumm]

The <pre> tag whitespace handling reported in #6437 was largely fixed. There's one detail though that's incorrect: Svelte seems to be adding an extra whitespace when there's a new line at the beginning when this does not happen using plain HTML unless there's an explicit white space after the <pre> tag.

Originally posted by @pablo-abc in #6437 (comment)