[FIX] web_editor: properly paste pre in pre

Commit [1] attempted to fix unexpected behavior when copy-pasting from a
<pre> to a <pre>, where the pasted text was smaller the intended, and
preceded and followed by new lines. The chosen approach had uninteded
side effects though as it removed any empty paragraph after the <pre>.
The source of the issue it tried to fix was that the pasted <pre> was
inserted as a new <pre>, splitting the existing element in two in the
process, instead of simply inserting the text content.

[1] 449cacfe

task-3086603

closes odoo/odoo#107826

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>