Add linter support for optional if statements

[nuwang]

We have a use case where a rule should be overridden without having to respecify the if condition. Currently, the absence of the if condition will trigger a lint failure because the if field is mandatory in the pydantic schema:

destinations:
  _pulsar_destination:
    rules:
      - id: pulsar_destination_docker_rule
        if: entity.params.get('docker_enabled')
        params:
            docker_set_user: '1000'
  pulsar-azure-gpu:
    inherits: _pulsar_destination
    rules:
      - id: pulsar_destination_docker_rule
        # the if condition is inherited here, so linter should not warn
        params:
            docker_volumes: $job_directory:ro,$tool_directory:ro,$job_directory/outputs:rw,$working_directory:rw,/data/alphafold_databases:/data:ro

[cat-bro]

This is not just linter warning. A rule without an if is no longer tolerated and all jobs fail.

Oct 15 00:27:41 dev-handlers galaxyctl[2588]: pydantic_core._pydantic_core.ValidationError: 1 validation error for TPVConfig
Oct 15 00:27:41 dev-handlers galaxyctl[2588]: destinations.pulsar-qld-gpu-dev.if
Oct 15 00:27:41 dev-handlers galaxyctl[2588]:   Field required [type=missing, input_value={'id': 'pulsar_destinatio...ct': False, 'env': None}, input_type=dict]
Oct 15 00:27:41 dev-handlers galaxyctl[2588]:     For further information visit https://errors.pydantic.dev/2.11/v/missing

This is unlikely to be a big deal for Galaxy Australia because we know about it, not sure if it is common for people to be using rule inheritance.