Map missing values to null

src/ext/java/org/opentripplanner/ext/legacygraphqlapi/datafetchers/LegacyGraphQLStoptimeImpl.java

@@ -110,6 +110,10 @@ private TripTimeOnDate getSource(DataFetchingEnvironment environment) {
     return environment.getSource();
   }
 
+  /**
+   * Generally the missing values are removed during the graph build. However, for flex
+   * trips they are not and have to be converted to null here.
+   */
   private Integer missingValueToNull(int value) {
       if(value == StopTime.MISSING_VALUE) {
         return null;