The com.opensymphony.xwork2.conversion.impl.StringConverter class, since Struts 2.5.11, incorrectly handles arrays containing null; it converts them to arrays containing the string "null" (using String.valueOf).
This is breaking one of our applications; some pages are OK, but on other pages, every non-provided field is populated with the String "null".
This is a similar issue to
WW-4737, but occurring in different code.
Please ensure that conversion to String always uses the two-argument version of Objects.toString, which preserves nulls, rather than String.valueOf, which converts them to the four-letter string "null".