First paragraph is sometimes not properly detected

[Sebobo]

This happens due to the Yoast Paper at some point adding empty <p> tags to some whitespaces and/or line endings.
Then the regular expression for the first paragraph cannot find the proper element anymore.

paper.getText() then returns markup like this:

    \n<p></p></header>\n<p>
<main class='site__content'></main></p>\n

Even though the input markup doesn't contain those tags.