Do not error when hosp_death_risk is NA without hospitalisation

[joshwlambert]

This PR addresses an issue whereby the sim_linelist() function would error when hosp_death_risk is NA and onset_to_death is specified as a delay distribution by the user (i.e. not NA). See example sim_linelist() call that would produce this error.

sim_linelist(
  contact_distribution = contact_distribution,
  infectious_period = infectious_period,
  onset_to_hosp = NA,
  onset_to_death = onset_to_death,
  onset_to_recovery = onset_to_recovery,
  hosp_risk = NA,
  hosp_death_risk = NA,
  non_hosp_death_risk = 0.3,
  prob_infection = 0.5
)

However, this is unwanted behaviour as in this parameterisation nobody in the simulation is hospitalised (onset_to_hosp = NA and hosp_risk = NA), so hosp_death_risk should be allowed to be NA. This PR resolves this by updating .cross_check_sim_input() to check whether hosp_risk is NA and if so not erroring.

Thanks to @CarmenTamayo for reporting this issue.