Handle the case if the group size is equal to 0 for the LCT model

[lenaploetzke]

Motivation / Current Behaviour

Currently, if the population size-deaths of an age group is equal to zero, a NaN value occurs in the simulations.

Enhancement description

This should be handled similarly to the ODE models, as e.g. in cpp/models/ode_secir/model.h. See also #1102. 1/(population size-deaths) should be set to zero.

Additional context

No response

Checklist

• Attached labels, especially loc:: or model:: labels.
• Linked to project