Fix NPE when trying to delete not existing queue

Both `queue.delete` and `exchange.delete` methods are idempotent.
In the case no queue is deleted, they behave as if an empty
queue/exchange was.

src/main/java/com/github/fridujo/rabbitmq/mock/MockNode.java

@@ -86,9 +86,9 @@ public AMQP.Queue.DeclareOk queueDeclare(String queueName, boolean durable, bool
     }
 
     public AMQP.Queue.DeleteOk queueDelete(String queueName, boolean ifUnused, boolean ifEmpty) {
-        MockQueue queue = queues.remove(queueName);
-        queue.notifyDeleted();
-        return new AMQImpl.Queue.DeleteOk(queue != null ? queue.messageCount() : 0);
+        Optional<MockQueue> queue = Optional.ofNullable(queues.remove(queueName));
+        queue.ifPresent(MockQueue::notifyDeleted);
+        return new AMQImpl.Queue.DeleteOk(queue.map(MockQueue::messageCount).orElse(0));
     }
 
     public AMQP.Queue.BindOk queueBind(String queueName, String exchangeName, String routingKey, Map<String, Object> arguments) {

src/test/java/com/github/fridujo/rabbitmq/mock/ChannelTest.java

@@ -7,6 +7,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Random;
+import java.util.UUID;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.TimeUnit;
@@ -168,6 +169,17 @@ void exchangeDeleteNoWait_removes_it() throws IOException, TimeoutException {
         }
     }
 
+    @Test
+    void exchangeDelete_does_nothing_when_not_existing() throws IOException, TimeoutException {
+        try (Connection conn = new MockConnectionFactory().newConnection()) {
+            try (Channel channel = conn.createChannel()) {
+                assertThat(channel.exchangeDelete(UUID.randomUUID().toString())).isNotNull();
+                assertThat(channel.exchangeDelete(UUID.randomUUID().toString(), false)).isNotNull();
+                channel.exchangeDeleteNoWait(UUID.randomUUID().toString(), false);
+            }
+        }
+    }
+
     @Test
     void exchangeBind_binds_two_exchanges() throws IOException, TimeoutException {
         try (Connection conn = new MockConnectionFactory().newConnection()) {
@@ -292,6 +304,17 @@ void queueDeleteNoWait_deletes_it() throws IOException, TimeoutException {
         }
     }
 
+    @Test
+    void queueDelete_does_nothing_when_not_existing() throws IOException, TimeoutException {
+        try (Connection conn = new MockConnectionFactory().newConnection()) {
+            try (Channel channel = conn.createChannel()) {
+                assertThat(channel.queueDelete(UUID.randomUUID().toString())).isNotNull();
+                assertThat(channel.queueDelete(UUID.randomUUID().toString(), false, false)).isNotNull();
+                channel.queueDeleteNoWait(UUID.randomUUID().toString(), false, false);
+            }
+        }
+    }
+
     @Test
     void queuePurge_removes_all_messages() throws IOException, TimeoutException {
         try (Connection conn = new MockConnectionFactory().newConnection()) {
@@ -601,7 +624,7 @@ void commit_or_rollback_can_be_called_multiple_times_after_a_single_select() thr
                 channel.txCommit();
                 assertThat(channel.basicGet(queue, true)).isNotNull();
                 // Channel contained only one message as transactions are cleared after commit
-                assertThat(channel.basicGet(queue, true)).isNull(); 
+                assertThat(channel.basicGet(queue, true)).isNull();
 
                 channel.basicPublish("", queue, null, "third message".getBytes());
                 assertThat(channel.basicGet(queue, true)).isNull();