INT-3859: Fix NPE in the ImapMailReceiver

JIRA: https://jira.spring.io/browse/INT-3859

The Java Mail `MessageCache.getMessageBySeqnum()` has the code:
````java
if (msgnum < 0) {		// XXX - < 1 ?
    if (logger.isLoggable(Level.FINE))
	logger.fine("no message seqnum " + seqnum);
        return null;
}
````
about which the `IMAPFolder.search` doesn't care:
````java
matchMsgs[i] = getMessageBySeqNumber(matches[i]);
````
therefore pops `null`s to the top for the `ImapMailReceiver`

* Fix `NPE` filtering the `Message[]` from `null` items

Note: its enough difficult to reproduce it because it isn't clear how we can end up with:
````java
if (seqnums[msgnum-1] > seqnum)
		break;		// message doesn't exist
````

in the `MessageCache`.
That's why there is no test-cases on the matter.

**Cherry-pick to 4.1.x**

Polishing

- Only create a new array if needed.
- Add a test case

spring-integration-mail/src/test/java/org/springframework/integration/mail/ImapMailReceiverTests.java

@@ -870,6 +870,53 @@ public class ImapMailReceiverTests {
 		assertTrue(exec.isShutdown());
 	}
 
+	@Test
+	public void testNullMessages() throws Exception {
+		Message message1 = mock(Message.class);
+		Message message2 = mock(Message.class);
+		final Message[] messages1 = new Message[] { null, null, message1 };
+		final Message[] messages2 = new Message[] { message2 };
+		final SearchTermStrategy searchTermStrategy = mock(SearchTermStrategy.class);
+		class TestReceiver extends ImapMailReceiver {
+
+			private boolean firstDone;
+
+			public TestReceiver() {
+				setSearchTermStrategy(searchTermStrategy);
+			}
+
+			@Override
+			protected Folder getFolder() {
+				Folder folder = mock(Folder.class);
+				when(folder.isOpen()).thenReturn(true);
+				try {
+					when(folder.getMessages())
+							.thenReturn(!this.firstDone ? messages1 : messages2);
+				}
+				catch (MessagingException e) {
+				}
+				return folder;
+			}
+
+			@Override
+			public Message[] receive() throws MessagingException {
+				Message[] messages = searchForNewMessages();
+				this.firstDone = true;
+				return messages;
+			}
+
+
+		};
+		ImapMailReceiver receiver = new TestReceiver();
+		Message[] received = receiver.receive();
+		assertEquals(1, received.length);
+		assertSame(message1, received[0]);
+		received = receiver.receive();
+		assertEquals(1, received.length);
+		assertSame(messages2, received);
+		assertSame(message2, received[0]);
+	}
+
 	private void setUpScheduler(ImapMailReceiver mailReceiver, ThreadPoolTaskScheduler taskScheduler) {
 		taskScheduler.setPoolSize(5);
 		taskScheduler.initialize();