Backport of Avoid decoding nil pointer in map walker into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #17048 to be assessed for backporting due to the inclusion of the label backport/1.15.

The below text is copied from the body of the original PR.

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Description

This PR fixes a bug with the mapwalk decoding logic used on the client agents for things like config entries and resolved service configs. Currently, if there is a struct stored within a map[string]interface{} with at least one field set and one pointer field unset/nil (such as EnforcingConsecutive5xx in the PassiveHealthCheck struct), the mapwalk logic here would attempt to call reflect.Value.Type() which is invalid for a nil pointer and we'd get reflect: call of reflect.Value.Type on zero Value.

Testing & Reproduction steps

Replication steps:

1. Set up a server and client agent
2. Register the following service config:

{
    "Kind": "service-defaults",
    "Name": "frontend",
    "Protocol": "http",
    "UpstreamConfig": {
        "Defaults": {
            "ConnectTimeoutMs": 15000,
            "PassiveHealthCheck": {
                "MaxFailures": 1000000
            },
            "MeshGateway": {}
        }
    }
}

3. Attempt to register the following service on the client agent:

{
    "id": "frontend",
    "name": "frontend",
    "port": 9090,
    "checks": [
        {
            "Name": "check-9090-is-open",
            "TCP": "127.0.0.1:9090",
            "Interval": "10s"
        }
    ],
    "connect": {
        "sidecar_service": {
            "checks": [
                {
                    "Name": "check-9191-is-open",
                    "TCP": "127.0.0.1:9191",
                    "Interval": "10s"
                }
            ],
            "proxy": {
                "upstreams": [
                    {
                        "destination_name": "backend",
                        "local_bind_port": 9191
                    }
                ]
            }
        }
    }
}

4. Registration should fail with reflect: call of reflect.Value.Type on zero Value

PR Checklist

• updated test coverage
• [ ] external facing docs updated
• appropriate backport labels added
• not a security concern

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Overview of commits

• 7df53af

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR