Add YangInstanceIdentifier.coerceParent()

Add YangInstanceIdentifier.coerceParent()

There are a number of YangInstanceIdentifier users who perform
checks (or otherwise ensure) that YangInstanceIdentifier they are
dealing with is non-empty and hence its getParent() method cannot
possibly return null. These users still interact with methods
which explicitly require a non-null YangInstanceIdentifier -- hence
it would be useful to provide a bridge method which performs this
validation.

Change-Id: I7acb4b04ccbe3c536b690573519171af32089d41
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>