Fix latex error when change-tracked deleted display math starts a new paragraph

Fix latex error when change-tracked deleted display math starts a new paragraph

LaTeX refuses to break a line when it is empty. But we have to start
a new line here, otherwise the whole displayed equation would be
typeset as it were inline with previous content. The solution is to
put a zero-length space just before the line break. Moreover, this
is the right thing to do, as it simulates the extra space that is
normally added in this circumstance.