Fix error in to_junction_tree method of MarkovModel [fixes #889].
Fix error in to_junction_tree method of MarkovModel [fixes #889].

In MarkovModel.py line:555, the clique_factors are sometimes empty but because whenan empty list is passed to factor_product it throws an error. This commit adds an extra check
to see it empty or not.

ankurankan/pgmpy,pgmpy/pgmpy,pgmpy/pgmpy,khalibartan/pgmpy,khalibartan/pgmpy,ankurankan/pgmpy