front: fix missing rolling stock images in train schedule list

We were calling formatTrainScheduleSummaries() before the rolling
stock list got fetched. As a result all rolling stocks in the
formatted train schedule summaries were null on first load.

Wait for the rolling stock list to be available before kicking off
the train schedule summaries lazy loading.

front/src/applications/operationalStudies/hooks/useLazyLoadTrains.ts

@@ -52,7 +52,7 @@ const useLazyLoadTrains = ({
   const [postTrainScheduleSimulationSummary] =
     osrdEditoastApi.endpoints.postTrainScheduleSimulationSummary.useLazyQuery();
 
-  const { data: { results: rollingStocks } = { results: [] } } =
+  const { data: { results: rollingStocks } = { results: null } } =
     osrdEditoastApi.endpoints.getLightRollingStock.useQuery({ pageSize: 1000 });
 
   const trainSchedulesById = useMemo(() => mapBy(trainSchedules, 'id'), [trainSchedules]);
@@ -103,7 +103,7 @@ const useLazyLoadTrains = ({
             packageToFetch,
             rawSummaries,
             trainSchedulesById,
-            rollingStocks
+            rollingStocks!
           );
 
           // as formattedSummaries is a dictionary, we replace the previous values with the new ones
@@ -115,10 +115,10 @@ const useLazyLoadTrains = ({
       setAllTrainsLoaded(true);
     };
 
-    if (infraId && trainIdsToFetch && trainIdsToFetch.length > 0) {
+    if (infraId && trainIdsToFetch && rollingStocks && trainIdsToFetch.length > 0) {
       getTrainScheduleSummaries(infraId, trainIdsToFetch);
     }
-  }, [infraId, trainIdsToFetch]);
+  }, [infraId, trainIdsToFetch, rollingStocks]);
 
   return {
     trainScheduleSummariesById,