My site is in its infancy and has a lot of empty categories as products are being added. What is the best approach SEO-wise for these practically empty category pages? Should I hide the links to them until they have content? Or let the pretty much blank page get indexed? I will have a sitemap that keeps updated obviously.
Every page on your website should serve a purpose, it sounds like your empty categories serve no purpose at the moment, unless maybe they describe the products that are coming soon?.
If they don't add any value to users then remove them from the site and sitemaps, if they provide value you may as well include them and update them when necessary.
You can do one more thing :
<META NAME="ROBOTS" CONTENT="NOINDEX">
This will let them be visible and as well avoid them from being indexed by search engines. Once they have content this meta property can be removed.