Fix invalid SBML in PEtab/PySB import (#1433)
Fix invalid SBML in PEtab/PySB import (#1433)

The generated dummy SBML model did not pass SBML validation. This fixes it.
AMICI-developer/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI