Fix crash in trainee allocations when there are not solo chaired debates

The logger.info will throw if cost_matrix is an empty array; I figure may as well just skip the whole process at that stage though.

tabbycat/adjallocation/hungarian.py

@@ -170,8 +170,7 @@ def allocate(self):
             logger.info("allocating to %s: %s (c), %s", aa.debate, aa.chair, ", ".join([str(p) for p in aa.panellists]))
 
         # Allocate trainees, one per solo debate (leave the rest unallocated)
-
-        if len(trainees) > 0:
+        if len(trainees) > 0 and len(solo_debates) > 0:
             allocation_by_debate = {aa.debate: aa for aa in alloc}
 
             logger.info("costing trainees")