[BUGFIX] FormViewHelper option: group default

see:
https://github.com/FluidTYPO3/flux/pull/692
https://github.com/FluidTYPO3/flux/issues/691
Just a nit-pick, but this line should be `if (TRUE === empty($group))` so that it reads "if group is empty" rather than "if if is not false that group is empty". Everything else looks good.