Return null by default on getimage

pmd-core/src/main/java/net/sourceforge/pmd/lang/ast/AntlrBaseNode.java

@@ -90,6 +90,11 @@ public class AntlrBaseNode extends ParserRuleContext implements AntlrNode {
         }
     }
 
+    @Override
+    public String getImage() {
+        return null;
+    }
+
     @Override
     public int jjtGetNumChildren() {
         return children == null ? 0 : children.size();