This function makes two checks:
.check_external_consistency(external, periods = periods)An .eq_as_tb() tibble with external variables.
The periods of the baseline model.
The exogenous variable is a constant that is repeated over time;
The exogenous variable has exactly the same length as the shock.
Furthermore, it throws a warning that using exogenous series in a shock can lead to unexpected behavior if the length of the shock is not the same as the periods in the scenario.