add Nullable annotation

Author: vladak

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

@@ -275,7 +275,7 @@ public PathAccepter getPathAccepter() {
         return new PathAccepter(getIgnoredNames(), getIncludedNames());
     }
 
-    private String getCanonicalPath(String s) {
+    private @Nullable String getCanonicalPath(String s) {
         if (s == null || s.isEmpty()) {
             return null;
         }