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

test/transform/resource/before/BuilderSingularNullBehavior1.java

@@ -0,0 +1,13 @@
+import java.util.List;
+import java.util.Collection;
+
+import lombok.Singular;
+import lombok.Singular.NullCollectionBehavior;
+
+@lombok.Builder
+class BuilderSingularNullBehavior1 {
+	@Singular private List<String> names;
+	@Singular(nullBehavior = Singular.NullCollectionBehavior.JDK) private List<String> locations;
+	@Singular(nullBehavior = NullCollectionBehavior.GUAVA) private List<String> whatevers;
+	@Singular(nullBehavior = lombok.Singular.NullCollectionBehavior.IGNORE) private List<String> doohickeys;
+}