add early exits in .model_param_name_key()

[EmilHvitfeldt]

Follow up on tidymodels/tune#1071.

If there isn't any tuning parameters set in a workflow, we can early exit, which for the following example leads to an almost 20% speed increase

After

library(tidymodels)
ames <- ames |>
  select(where(is.numeric))

svm_spec <- null_model(mode = "regression")

set.seed(1)
cv1 <- rsample::bootstraps(ames)

set.seed(1)

bench::mark(
  run = fit_resamples(
    svm_spec,
    Sale_Price ~ .,
    cv1, metrics = metric_set(yardstick::rmse),
    control = control_grid(save_pred = TRUE)
  ), iterations = 10
)
#> Warning: Some expressions had a GC in every iteration; so filtering is
#> disabled.
#> # A tibble: 1 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 run           378ms    388ms      2.41     162MB     10.4

^{Created on 2025-08-01 with reprex v2.1.1}