Can a spontaneous caster use higher level slots to cast non-heightened lower level spells?

I’m an Oracle. I’ve picked up Divine Access (Ragathiel) which adds True Strike to my spell list. I’ve also added it to my spell repertoire as a 1-st level spell.

Since it is not one of my signature spells I can’t heighten it (not that it can be heightened to begin with).

I’m out of 1-st level spell slots. Can I use a 2-nd level spell slot to cast True Strike?