Fixing small issue with model propagation if it's run on a model with no genes.
Fixing small issue with model propagation if it's run on a model with no genes.

cshenry/fba_tools,cshenry/fba_tools,cshenry/fba_tools,cshenry/fba_tools,cshenry/fba_tools,cshenry/fba_tools,cshenry/fba_tools,cshenry/fba_tools,cshenry/fba_tools