Function FLEncoder_WriteUndefined 

pub unsafe extern "C" fn FLEncoder_WriteUndefined(
    arg1: FLEncoder,
) -> bool

Writes an undefined value to an encoder. (Its value when read will not be a NULL pointer, but it can be recognized by FLValue_GetType returning kFLUndefined.) @note The only real use for writing undefined values is to represent “holes” in an array. An undefined dictionary value should be written simply by skipping the key and value.