Merging of statistics must cope with execution time stats being unavailable

Closes #1380.

Author: gollux

cms/grading/steps/stats.py

@@ -103,19 +103,36 @@ def merge_execution_stats(
     if first_stats is None:
         return second_stats.copy()
 
+    def safe_sum(x, y):
+        if x is None:
+            return y
+        elif y is None:
+            return x
+        else:
+            return x + y
+
+    def safe_max(x, y):
+        if x is None:
+            return y
+        elif y is None:
+            return x
+        else:
+            return max(x, y)
+
     ret = first_stats.copy()
     ret["execution_time"] += second_stats["execution_time"]
 
     if concurrent:
         ret["execution_wall_clock_time"] = max(
             ret["execution_wall_clock_time"],
             second_stats["execution_wall_clock_time"])
-        ret["execution_memory"] += second_stats["execution_memory"]
+        ret["execution_memory"] = safe_sum(ret["execution_memory"],
+                                           second_stats["execution_memory"])
     else:
         ret["execution_wall_clock_time"] += \
             second_stats["execution_wall_clock_time"]
-        ret["execution_memory"] = max(ret["execution_memory"],
-                                      second_stats["execution_memory"])
+        ret["execution_memory"] = safe_max(ret["execution_memory"],
+                                           second_stats["execution_memory"])
 
     if first_stats["exit_status"] == Sandbox.EXIT_OK:
         ret["exit_status"] = second_stats["exit_status"]