fix empty matches error

app/corpora/cqi/models/subcorpora.py

@@ -47,6 +47,8 @@ class Subcorpus:
 
     def dump_values(self, context=25, first_result=0,
                     num_results=float('inf')):
+        if self.size == 0:
+            return {"matches": []}
         first_result = max(0, first_result)
         last_result = min((first_result + num_results - 1), (self.size - 1))
         matches = []