Fixup _has_start_at method

Need to use .get() to access a thing that might not exist.

Also use _timestamp because I think it's more widely used.

backdrop/core/data_set.py

@@ -117,14 +117,14 @@ def execute_query(self, query):
             if shift != 0:
                 return self.execute_query(query.get_shifted_query(shift))
 
-        if not query.sort_by and _has_start_at(data):
-            return sorted(data.data(), key=lambda d: d['_start_at'])
+        if _has_timestamp(data) and (not query.sort_by):
+            return sorted(data.data(), key=lambda d: d['_timestamp'])
 
         return data.data()
 
 
-def _has_start_at(data):
-    return all([data_point['_start_at'] for data_point in data.data()])
+def _has_timestamp(data):
+    return all([data_point.get('_timestamp') for data_point in data.data()])
 
 
 def build_data(results, query):