It is "probably" ok.
Is there any chance that you can put off getting this work done until after your first trimester? Or at least until you know if you are PG or not? If not, I'd suggest getting only the novacaine as it is a local anesthetic and it is better to have dental work done than to ignore it and possibly get an abscess or have an infection that spreads throughout the body.
The only studies I've seen/read about laughing gas was one back in the nineties that showed that repeated exposure lead to fertility problems, but that was in dental hygenists and dentists. So, most likely, being exposed a couple of times won't do anything, but that is entirely up to you what you are comfortable with.
Homebirth Midwifing mama to five blessings in Northeastern PA.