Bug 1984, Bug 2005: Changed valueEquals to work with null values.

ImmutableNode implementation hashCode and equals did not worked
properly with null values, which are rare, but allowed by
typedef empty statement.

Methods were fixed to work properly with null values and
added comment which explicitly says, that nulls are allowed
because of empty type.

Change-Id: I8fa3431bae19783dfbc0e2d3684c2e981e11da46
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
(cherry picked from commit 7f0c1aa806f952584ac71feecbc17f3834705510)