Add tests to ensure `EnumSet` and `EnumMap` work correctly with "null-as-empty"

[cowtowncoder]

While null-handling with @JsonSetter is tested to a degree, Enum collections (EnumSet, EnumMap) have quite particular handling (due to JDK implementation details), and should probably be separately and explicitly tested.

[cowtowncoder]

Looks like handling by EnumMap is correct, but EnumSet needs to be tweaked: both wrt empty value for itself and handling of null as contents (will not be possible to "use empty", but should be possible to skip).