support empty sboterm in reactions

xbanalysis/model/xba_reaction.py

@@ -15,7 +15,7 @@ class XbaReaction:
     def __init__(self, s_reaction, species, functions, compartments):
         self.id = s_reaction.name
         self.name = s_reaction.get('name', self.id)
-        if 'sboterm' in s_reaction:
+        if ('sboterm' in s_reaction) and (type(s_reaction['sboterm']) is str):
             self.sboterm = SboTerm(s_reaction['sboterm'])
         self.reaction_string = s_reaction['reactionString']
         self.reversible = s_reaction['reversible']