common: Add external annotations to NavigableSet

common: Add external annotations to NavigableSet

The following methods in NavigableSet:

ceiling()
floor()
higher()
lower()

can all return null if there is no corresponding element in the set,
independently of the container type declaration. They should be
marked @Nullable.

Change-Id: I66b9cd5ba8b6eafc69032fb0ae27a9d1ce9e464c
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/59296
Reviewed-by: Hudson CI