Clarify policy-forwarding and routing-policy if no rules are present

[dplore]

Change Scope

• Clarify policy-forwarding such that if no rules are present, all packets will be matched

[dplore]

Related to discussion on #1150

[OpenConfigBot]

No major YANG version changes in commit def87eb

[s19nal]

LGTM - updated clarification reads well.

[dplore]

We discussed this a second time in the OC Operators meeting on Nov 26, 2024.
It is observed that policy-forwarding and routing-policy should have the same semantics. At the moment, both are ambiguous about the behavior when there are no rules (policy-forwarding) or no conditions (routing-policy) defined.

For routing-policy, vendor native implementations widely support policies which have no 'conditions' matching all packets.

Example native routing policy configuration: https://gist.github.com/dplore/cff9a4c782f68734adaafff8952fc3cb (Kudos to @oscargdd for sharing this investigation)

So I am updating this description to assert that no rules means match all packets.

[xuqma]

LGTM for BGP routing policies. Thanks Darren.

[dplore]

After a bit more discussion, there was a request to also clarify what happens if there is a routing-policy statement with no condition AND no action, as well as a policy-definition without any statements. These should be treated as errors (grpc error: INVALID_ARGUMENT, netconfig error missing-element).

Note that policy-result is an action. So one may write a valid OC routing-policy which has for example, one statement which as no condition (matching all routes) and a policy-result of ACCEPT_ROUTE.

I will update this pull request accordingly.