added missing supress statement in Tb culture class

src/tlo/methods/tb.py

@@ -2307,7 +2307,7 @@ def apply(self, person_id, squeeze_factor):
 
             # Return the footprint. If it should be suppressed, return a blank footprint.
             if self.suppress_footprint:
-                return
+                return self.make_appt_footprint({})
             else:
                 return ACTUAL_APPT_FOOTPRINT