Change is in rawLaTeXInline in LaTeX reader, but
it affects the markdown reader and other readers
that allow raw LaTeX.
Previously, trailing `{}` would be included for
unknown commands, but not for known commands.
However, they are sometimes used to avoid a trailing
space after the command. The chances that a `{}`
after a LaTeX command is not part of the command
are very small.
Closes#5439.
