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

test/transform/resource/before/BuilderSingularMapsWithSetterPrefix.java

@@ -1,6 +1,7 @@
 //FORMAT: javaLangAsFQN = skip
 //FORMAT: generated = skip
 //FORMAT: finalParams = skip
+//CONF: lombok.singular.nullCollections = ignore
 import java.util.Map;
 import java.util.SortedMap;