Transport source must include zeros (or other values) when two tracers are considered (seen on Ubuntu machines only)

[smolins]

To run the transport demo of the short course we had to add a source of zeros for tracer2, when we had a tracer1 in the rainwater so that it could be run in the Ubuntu container. Development on Mac was not throwing out error. Test example below:

https://github.com/amanzi/ats-short-course/blob/ats-short-course-20250908/03_transport/.transport_solution.xml