[singular][issue #2221] the plural builder method now nullchecks its argument with configurable results.

test/transform/resource/after-ecj/BuilderWithToBuilder.java

@@ -28,6 +28,8 @@ import lombok.Builder;
       return this;
     }
     public @java.lang.SuppressWarnings("all") BuilderWithToBuilder.BuilderWithToBuilderBuilder<T> bars(final java.util.Collection<? extends T> bars) {
+      if ((bars == null))
+          throw new java.lang.NullPointerException("bars cannot be null");
       if ((this.bars == null))
           this.bars = new java.util.ArrayList<T>();
       this.bars.addAll(bars);