Improve comment about plan being NULL

per suggestion from Jelte Fennema-Nio.

src/backend/distributed/planner/function_call_delegation.c

@@ -525,7 +525,12 @@ ShardPlacementForFunctionColocatedWithDistTable(DistObjectCacheEntry *procedure,
 
 		if (partitionParam->paramkind == PARAM_EXTERN)
 		{
-			/* Don't log a message, we should end up here again without a parameter */
+			/*
+			 * Don't log a message, we should end up here again without a
+			 * parameter.
+			 * Note that "plan" can be null, for example when a CALL statement
+			 * is prepared.
+			 */
 			if (plan)
 			{
 				DissuadePlannerFromUsingPlan(plan);