Deserialization from blank (not empty) String fails for Collections

[cowtowncoder]

(note: follow-up to #458

Looks like deserialization of List<POJO> (wrapped) succeeds from empty String (zero-length), but not from blank (all white-space). Since latter case is a common occurrence for indented XML content, it should behave the same.
Handling is actually implemented in jackson-databind but since it only affects XML backend (with default coercion settings, that is), I'll file issue here, and add unit test. Fix itself will be in databind however.