Binding: no point to set target value if got false
Binding: no point to set target value if got false

If we return false, GBinding ignores target GValue, so there’s no point
setting it. Besides, if the transform function returned false, that says
they couldn't calculate a target value, so they shouldn’t assign to `to`

I default-construct primitive `T_to`s in case anyone *was* reading those
for some daft reason, so they get a zero-initialised value instead of UB

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm