You can't cast a Flameblast without a target at all, nor can I think of any reason you would want to. If your target moves or otherwise becomes illegal after you aim the spell at them, then the spell is cancelled, but you can't just choose to cast a spell aimed at nothing. Similarly, you can't choose to make an attack aimed at nothing, as far as I can tell, within the framework of the rules.
According to sweeping within the codex, it's not granting you the option of a second attack. It never says the word "may" or any other terms to indicate it is optional. Therefore, the second attack is necessary. You've already decided that you want to make both attacks whenever you declare that you are using a sweeping attack in the first place. Thus, you must finish making both attacks if at all possible, and having friendly creatures (including yourself) in the zone makes it possible.
As a side note with the Flameblast scenario, if somebody tries to cast Flameblast or some such and accidentally misreads the range as further than it actually is, then they are not required to hit themselves in the face with it when you point out their mistake and the lack of enemy targets within range. They can choose to simply not cast the spell, as per the rules on page 13 of the English Rulebook v3.
Thematically, I agree that it doesn't make much since that you would be required to hit yourself in the face with it. However, I have been unable to find anything in the rules that would support a decision not to use the second attack. It's just not worded in a way as to make it optional or to make friendly targets invalid. So, for now at least, if you want to use sweeping you have to be prepared to hit two targets even if that means hitting yourself. Unless I've missed something.