lib: prevent gRPC assert on missing YANG node

lib: prevent gRPC assert on missing YANG node

`yang_dnode_get` will `assert` if no YANG node/model exist, so lets test for
its existence first before trying to access it.

This `assert` is only acceptable for internal FRR usage otherwise we
might miss typos or unmatching YANG models nodes/leaves. For gRPC usage
we should let users attempt to use non existing models without
`assert`ing.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>