initiailizing with a draws or fit object fails if the model is created without a .stan file

[n-kall]

Describe the bug
If a model is created without a .stan file, that is only with an exe_file, using another fit object (or draws) as an init fails with an uninformative error message.

To Reproduce

library(cmdstanr)

model_code <- "data {
 int N;
 array[N] real x;
 }
parameters {
  real mu;
}
model {

  // likelihood including constants
  x ~ normal(mu, 1);
}

"

model <- cmdstan_model(write_stan_file(model_code), exe_file = "model_bin")

model_nostanfile <- cmdstan_model(exe_file = "model_bin")

d <- list(x = c(1,2,3,4,5), N = 5)

pth <- model$pathfinder(
  data = d
)

# works fine
mcmcfit <- model$sample(
  data = d,
  init = pth
)

# fails  Error: 'init' contains empty lists.
mcmcfit_nostanfile<- model_nostanfile$sample(
  data = d,
  init = pth
)

Expected behavior
The sampling succeeds, or a more informative error message is given.

Operating system
Linux (Ubuntu 24.04.2)

CmdStanR version number
0.9.0

Additional context
I believe this is due to model_nostanfile$variables() failing to determine which parameters are in the model, but I think passing all the variable inits without determining which variables are actually in the model should still work