It doesn't apply to all 9s of course but there's definitely a correlation. It's not that it's like this in every area of their life either, but I'm sure a lot of 9s have to come to terms with their ”nothingness”, in some significant area at some point in their life.
I really like how my dad, who is an ISFJ 9, put his share of this cake when I was little:
”My one talent is that I'm not bad at anything. I'm not good at anything either, but I know a little bit about everything. So I never shine, but I never suck either.”
He really is a ”jack of all trades but a master of none”. I've noticed ISFJ 9's seem to be prone to this, probably because Si is generally good at learning existing systems and the ”know how” but 9s at average levels often lack focus and/or ambition.
I'm also a 9, but an INFJ, which is probably a part of why I'm different from my dad in this. For me it's more of a people thing: I merge with others to the point I sort of nullify myself.
What this leads to is pretty unavoidable: At some point in his youth my dad started thinking he wasn't good at anything. And at some point I started thinking I basically had no personality, nothing to draw anyone toward me.
Whatever way this appears in you, there's only one way to conquer it: start treating it as a trait, not a lack of one.
It took me forever to realize this, but it has been one of the most beneficial things I've learned in the past few years.
The first step is just to look at yourself differently: For my dad to realize he wasn't untalented: his talent was not being bad at anything. And for me to realize I did have personality, that I didn't just dissapear into others: it is a personality trait to be adaptive.
Once you realize something is a trait, and not a lack of one, you can find a way to be active. 9s are always going to be passive by nature, but I guess the best thing we can do is to start being active about that passiveness, if that makes sense. To make it so that it's not something that just happens to you: It's something that you control. It's just as valid strategy as anything else.
If you're like my dad, then take everything out of your skills. If you're not bad at anything, you're already pretty good at things. But I think it's still important to strive to be better. You're not stuck. Even if you're already an all-rounder, you can always become even better at that. Or you could still strive for something you care about more than some other things.
If you're like me, then you have to be the one to control when, where, how, and why you merge with others. Even if you already see the benefits of being passive, your goal shouldn't be just peace and harmony. Keep your own agenda in check, and you can become really effective at things like solving conflict and improving people's communication. You shouldn't supress your active side so give it a chance to act on its own too.
I also know 9s who think their one gift to the world is basically that they don't bother anyone, that they don't do anything wrong. If you're like this, then treat it as a great foundation to do actual good. Sometimes you have to bother people a little to be able to help them on a larger scale but if you get something from not bothering people, then you'll definitely get more from actually helping them.
I don't remember who said it, but I really like the thought that being a 3 is the best thing a 9 can do.