add Nullable annotation

Author: vladak

opengrok-indexer/src/main/java/org/opengrok/indexer/util/Executor.java

@@ -131,7 +131,7 @@ public Executor(List<String> cmdList, @Nullable File workingDirectory, Map<Strin
      * @param workingDirectory The directory the process should have as the working directory
      * @param useTimeout terminate the process after default timeout or not
      */
-    public Executor(List<String> cmdList, File workingDirectory, boolean useTimeout) {
+    public Executor(List<String> cmdList, @Nullable File workingDirectory, boolean useTimeout) {
         this(cmdList, workingDirectory);
         if (!useTimeout) {
             this.timeout = 0;