There are multiple ways to determine the opposite of nihilism. It depends on what is being focused on.
According to the question, however, the word being looked for here is described by the following:
Someone who has specific convictions, positive or negative, on a given subject.
The opposite of somebody who believes in nothing, evidence of not, is somebody who believes in something, evidence or not.
To believe in something despite a lack of evidence (or to follow it blindly with evidence) is to have faith.
Merriam-Webster gives the following non-religious senses of the word (I only give the non-religious senses, since it was asked to focus on that):
2 b(1) : firm belief in something for which there is no proof
// clinging to the faith that her missing son would one day return
2 b(2) : complete trust
3 : something that is believed especially with strong conviction
: without question
// took everything he said on faith
// Nick wiped at the moustache of sweat droplets that was as much a part of his face as his eyes and nose and gave a shrug that indicated a certain lack of faith in our judgment.
— Tom Perrotta, Joe College, 2000
// Claypool appreciated the faith offensive coordinator Chip Long showed in calling his number after the receiver dropped a short pass on the first play of the drive.
— Mike Berardino, Indianapolis Star, "Notre Dame WR Chase Claypool 'needs the ball more,' says ACC Network analyst Eric Mac Lain," 8 Nov. 2019
Of course, if you have faith, you are faithful, and something like the following could be said:
Despite all evidence to the contrary, she remained faithful to the conviction that her father was innocent.
In terms of the example sentences in the question, the actual noun faithful might have too many religious connotations. So, it would probably sound better to use the adjective or the noun faith instead:
Unlike nihilists, those with faith have specific beliefs.
The decline in active investing suggests there are fewer faithful traders than ever in the market.
As an aside, I came across a different interpretation of the opposite of nihilism—one that isn't asked for in the question, but which I still found noteworthy.
The following excerpt comes from "Existentialism, Bounded in a Nutshell: The Basic Philosophical Concepts," a lecture note at the University of Idaho:
3) Existentialism is not nihilism:
"Nihilism" is the belief that nothing matters. Existentialism is the attempt to confront and deal with meaninglessness...to not succumb to nihilism or despair: to not give up or avoid responsibility.
For Camus, the entire purpose of Existential philosophy is to overcome absurdity, or, more accurately, for man to triumph over the absurdity of existence.
So Existentialism is the opposite of nihilism: the nihilist says "There is no god, no heaven or hell, so screw it: there can be no right or wrong. Let's party!" The Existentialist says "There is no god, no heaven or hell, so you and I alone must figure out to make life meaningful and good -- we must, in fact, work without cosmic aid to figure out what 'good' itself is."
In that interpretation of opposites, the opposite of a nihilist is an existentialist.