If a character is out of 1st level slots, but has a 2nd level slot, can they still cast a spell at first level?

I know that 1st level spells can be cast using 2nd level slots. If a character is out of 1st level slots, but has a 2nd level slot, can they still cast a spell at first level, or are they forced to make the spell hit harder? I’d allow it

— Mike Mearls (@mikemearls) August 28, 2018