add Nullable annotation

Author: vladak

opengrok-indexer/src/main/java/org/opengrok/indexer/configuration/Project.java

@@ -34,6 +34,7 @@
 import java.util.logging.Logger;
 import java.util.regex.PatternSyntaxException;
 
+import org.jetbrains.annotations.Nullable;
 import org.jetbrains.annotations.VisibleForTesting;
 import org.opengrok.indexer.logger.LoggerFactory;
 import org.opengrok.indexer.util.ClassUtil;
@@ -548,6 +549,7 @@ public static Project getProject(String path) {
      * @param file the file to lookup
      * @return the project that this file belongs to (or {@code null} if the file doesn't belong to a project)
      */
+    @Nullable
     public static Project getProject(File file) {
         Project ret = null;
         try {