Capabilities unavailable for yang models without revision

Capabilities unavailable for yang models without revision

The updated logic now gracefully handles optional revisions
by defaulting to an empty string ("") when the revision is not present.
This change ensures that capabilities for Yang models without
a specified revision can be retrieved successfully, aligning
with modifications in the netconf module that now require an
empty string to represent models without a revision.

JIRA: NETCONF-1263
Change-Id: I65da21f877f572cc227ac5ec1ca8c76bf66a8b77
Signed-off-by: Yaroslav Lastivka <yaroslav.lastivka@pantheon.tech>