Fix AMQP09 panic when exchangeDeclareArgs is nil - issue #3576

[Copilot]

Problem

The AMQP09 output would panic with "assignment to entry in nil map" when configured with exchange_declare.arguments. This occurred because the exchangeDeclareArgs field was never initialized before attempting to assign values to it.

output:
  amqp_0_9:
    urls:
      - amqp://guest:guest@localhost:5672/
    exchange: my-exchange
    exchange_declare:
      enabled: true
      type: direct
      durable: true
      arguments:
        alternate-exchange: my-ae
        x-message-ttl: 60000

The above configuration would cause a panic at runtime when the code tried to execute:

a.exchangeDeclareArgs[key] = value  // panic: assignment to entry in nil map

Solution

Added a length check before initializing the exchangeDeclareArgs map. The map is now only initialized when there are actually arguments to set:

if len(args) > 0 {
    a.exchangeDeclareArgs = make(amqp.Table)
    for key, value := range args {
        a.exchangeDeclareArgs[key] = value
    }
}

This preserves the original behavior where exchangeDeclareArgs remains nil when no arguments are provided (which is valid for AMQP exchange declarations), while properly initializing the map when arguments exist.

Testing

Added comprehensive unit tests to cover both scenarios:

• Configuration with exchange arguments (previously panicked, now works)
• Configuration without exchange arguments (preserves original nil behavior)

All existing integration tests continue to pass, ensuring no regression.

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