Null pointer exception on missing headers using RabbitMQ instrumentation

[mellis3000]

Describe the bug
Similar to #4902, we are getting null pointers for what seems to be missing headers for the request, looking at the stack trace. In our case, the null pointer is thrown in RabbitDeliveryAttributesGetter.java instead of DeliveryRequestGetter.java as in the linked issue which was fixed by adding a null check https://github.com/open-telemetry/opentelemetry-java-instrumentation/pull/4903/files

Steps to reproduce
Consume a RabbitMQ event without headers

What did you expect to see?
Proceeding by ignoring missing headers instead of throwing java.lang.NullPointerException

What did you see instead?

Consumer io.opentelemetry.javaagent.instrumentation.rabbitmq.TracedDelegatingConsumer@2d2c535a (***) method handleDelivery for channel AMQChannel(amqp://*****@***.***.***.***:5672/,1) threw an exception for channel AMQChannel(amqp://*****@***.***.***.***:5672/,1)
java.lang.NullPointerException: null
        at io.opentelemetry.javaagent.instrumentation.rabbitmq.RabbitDeliveryAttributesGetter.header(RabbitDeliveryAttributesGetter.java:93)
        at io.opentelemetry.javaagent.instrumentation.rabbitmq.RabbitDeliveryAttributesGetter.header(RabbitDeliveryAttributesGetter.java:14)
        at io.opentelemetry.javaagent.shaded.instrumentation.api.instrumenter.messaging.MessagingAttributesExtractor.onEnd(MessagingAttributesExtractor.java:111)
        at io.opentelemetry.javaagent.shaded.instrumentation.api.instrumenter.Instrumenter.doEnd(Instrumenter.java:224)
        at io.opentelemetry.javaagent.shaded.instrumentation.api.instrumenter.Instrumenter.end(Instrumenter.java:144)
        at io.opentelemetry.javaagent.instrumentation.rabbitmq.TracedDelegatingConsumer.handleDelivery(TracedDelegatingConsumer.java:76)
        at com.rabbitmq.client.impl.ConsumerDispatcher$5.run(ConsumerDispatcher.java:149)
        at com.rabbitmq.client.impl.ConsumerWorkService$WorkPoolRunnable.run(ConsumerWorkService.java:104)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
        at java.base/java.lang.Thread.run(Thread.java:834)

What version are you using?
v1.22.1

Environment
Compiler: AdoptOpenJDK OpenJDK 64-Bit Server VM 11.0.3+7
OS: Ubuntu 20.04