Fix uninitialized ExtensionDefinition for openconfig-version

Fix uninitialized ExtensionDefinition for openconfig-version

The StatementDefinition for openconfig-version extension is
not bound to any revision. When populating the ExtensionNamespace
with openconfig-version, we have to store it with a revisionless
QName key. This way we can properly extract the corresponding
ExtensionDefinition later down the road in the
UnknownEffectiveStatementBase.

This issue causes NPE in MDSAL's binding generator v2.

Change-Id: Ie24c09b56986c87b5118d715ec6c8e755cf07ebe
Signed-off-by: Igor Foltin <igor.foltin@pantheon.tech>