[FIX] LineBreak: List item empty with br are displayed as twice line

[Gorash]

The mobile versions represent the
in li as making 2 lines and not
just one (unlike browsers). The implementation of linebreaks needs to
change. The invisible terminal BR must be an invisible character so that
the previous BR is displayed but respecting more the W3 rules.

[Zynton]

Quid de <br>\u200B<fontawesome>\u200B alors?

[Gorash]

Parsing OK (font, container, empty...), rendering OK, but It's not good I think.

TODO later: Choice to be discussed because the <br> in the <li> whether it is for editing or not seems incorrect.
Therefore it is necessary to be able to add <p> in the li when there is at least one <br>.
Moreover, if there is a <p> during parsing, it must be kept.
(This is a bug regardless of the publisher and even without an editor)