My understanding is that a lack of robots.txt should be fine and the lack of a sitemap.xml shouldn't be too troublesome as long as there is something linking to your site and all pages are linked from somewhere on your site (the sitemap helps search engines find all the links in one place but you have a nav bar or an article list that should work similarly, but you can give a suggestion to search engines about how often they should recrawl in your sitemap, which I don't believe you can influence any other way).

There are many metrics and facets of a website considered for SEO, with varying weights. The absence of either a robots.txt or sitemap.xml has a non-negligible but relatively minor weight compared to some other metrics. The files should be present and accurate when optimizing for seo. Ruby on Rails for example creates an empty robots.txt file with new projects, it does not create any sitemap.xml however.

I’m also ignorant of this, and to add a question on top of yours: is it worth worrying about robots.txt for personal portfolio websites built from scratch?

Depends on your hosting platform.

Nope! A site without robots.txt is defaulted to saying, please do the default — crawl my site in its entirety.