Fix bug #8305: ^ symbol does not render into upward arrow when using mhchem

Fix bug #8305: ^ symbol does not render into upward arrow when using mhchem

The mhchem package treats the caret both as a shorthand for \uparrow or
as a superscript operator according to whether it is surrounded by
spaces or not. The \ce and \cf insets allow inserting spaces but there
is no provision for inserting a space after the caret, which is always
considered by LyX as a superscript operator. The solution here is to
insert a space after the caret if the superscript is empty or an empty
brace inset.

Mathed does not allow empty superscripts, so an empty brace has to be
inserted when working in LyX. On the other hand, when importing latex
code, an empty superscript is retained.

This has no effect whatsoever for normal latex code, as a space after
the caret is ignored. In any case, the output is only changed if an
empty brace inset is used as superscript. Specifically, the output is
changed from "^{{}}" to "^ {}".