ss: Add @NonNull to attribute name-getting methods

ss: Add @NonNull to attribute name-getting methods

If the quark is valid, its name and full path can never be null.

This removes a check in AttributeTree that would return null on failure.
Instead, let it throw an ArrayOutOfBoundsException if the quark is
invalid, like many other methods in that class already do.

Change-Id: I384ff8be26f003d684c9f9170cba2d155ed3c08e
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/44281
Reviewed-by: Hudson CI