only unref GObject is they're not null

Change-Id: I7704132f272fb6409ba606ce3a25903b3d2c84e0
bindings/java: better handling of reading java enums
bindings/java: avoid converting null objects
binginds/java: fixed parsing of read/write attribute of properties

Change-Id: Id074067ed525d89629671e3b6d93a9c30f42d442