Fix NullPointerException when a bound site transitions to a state with no defined binding reaction (#36).
When a bound site changed to a (state, partner-state) signature that had no associated binding reaction, the solver dereferenced a null reaction name / bond length and crashed. The bond now keeps its existing reaction identity and length, and its spontaneous-dissociation probability becomes 0 for the undefined state (equivalent to an unbinding rate of 0.0). Applied in both TransitionReactions and AllostericReactions, with a regression test.