BUG-650: fromYangInstanceIdentifier() can return null

Properly interpret a null return as an absent identifier.

Clarified cast in #toBinding(Entry) and explained
why it is necessary, even if Eclipse reports
it as unnecessary (difference in various java compilers).

Change-Id: I2dd7cd631cb958a28ddce8db62d20e0292ae8a39
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-650: fromYangInstanceIdentifier() can return null"