AMQP: Fixes handling null replyTo header in AmqpReplyToSinkStage

[Grandys]

Fixes situation when properties of the message are present, but replyTo header in this properties is null. In that case elem.properties.map(_.getReplyTo) evaluated to Some(null) and sending message resulted with the error:

java.lang.IllegalStateException: Invalid configuration: 'routingKey' must be non-null.
	at com.rabbitmq.client.impl.AMQImpl$Basic$Publish.<init>(AMQImpl.java:2280)
	at com.rabbitmq.client.AMQP$Basic$Publish$Builder.build(AMQP.java:1219)
	at com.rabbitmq.client.impl.ChannelN.basicPublish(ChannelN.java:696)
	at com.rabbitmq.client.impl.recovery.AutorecoveringChannel.basicPublish(AutorecoveringChannel.java:202)
	at akka.stream.alpakka.amqp.impl.AmqpReplyToSinkStage$$anon$1$$anon$2.onPush(AmqpReplyToSinkStage.scala:74)

[ennru]

LGTM.

[ennru]

Thank you @Grandys!