Haha, sure. I'm not sure "null particle" (or "zero particle") is an official term, but it's the one our instructor used way back in my time at the (now sadly defunct)
Japanese FALCON program. (I also forget whether or not it was actually in our textbook, Jorden's
Japanese: the Spoken Language, or whether it was supplementary information. Perhaps the former.)
Basically, as you point out, there are many cases where は and が are not strictly necessary. Going a step further than that, there are actually some cases and contexts where having no particle is preferred. This is called the "null" or "zero" particle rather than a "dropped" particle because the latter would imply that the particle is technically necessary and the "correct" version with this just being a slang/colloquial version of it with essentially the same nuance—which isn't the case. In certain contexts, the null/zero particle is the preferred particle, and using は or が would impart an awkward or undesirable nuance.
Some simple and easily understood examples would be その髪型、似合っていますよ or このビール、うまいよね.
In both of these sentences, the most natural to express the basic sentiment of "That hairstyle looks good on you" and "This beer tastes great" is with no particle. If you were to use は here, the nuance would be contrastive and potentially insulting, i.e. その髪型は似合っていますよ ("That hairstyle, at least, looks good on you {but your clothes, on the other hand... / but let's not talk about your face...}"), while が would be weirdly exhaustive and emphatic ("It's
your hairstyle that suits you" as if you were responding to the question "What is the one thing that suits me?") In this case, it's not that the "correct particle" は or が is being "dropped", it's that no particle (i.e. the zero/null particle) is the most natural option from the start.
As far as I know, many textbooks don't teach this, but I believe it's a somewhat important concept to understand at the higher levels if you're actually striving for relatively natural/near-native fluency in the spoken language.
(edited to add: Just for clarity, dropped/omitted particles in casual language are also a thing. That is to say, not every instance of no particle is necessarily the zero/null particle. The distinction is whether the presence or absence of the particle would fundamentally alter the nuance of the utterance.)
