be more robust when attempting to delete history

fixes #2837

Author: Vladimir Kotal

opengrok-web/src/main/java/org/opengrok/web/api/v1/controller/ProjectsController.java

@@ -207,6 +207,10 @@ public void deleteHistoryCache(@PathParam("project") String projectName) throws
         logger.log(Level.INFO, "deleting history cache for project {0}", projectName);
 
         List<RepositoryInfo> repos = env.getProjectRepositoriesMap().get(project);
+        if (repos == null || repos.isEmpty()) {
+            logger.log(Level.INFO, "history cache for project {0} is not present", projectName);
+            return;
+        }
 
         // Delete history cache data.
         HistoryGuru guru = HistoryGuru.getInstance();