Fix regression introduced by 11f9e9084b

Motivation:

While cherry-picked 11f9e9084b I changed the EmbeddedChannel implementation to not allow no ChannelHandlers when constructing it.
This was done by mistake.

Modifications:

Revert change and add unit test.

Result:

Restore old behavior.

transport/src/main/java/io/netty/channel/embedded/EmbeddedChannel.java

@@ -82,18 +82,6 @@ public class EmbeddedChannel extends AbstractChannel {
             throw new NullPointerException("handlers");
         }
 
-        int nHandlers = 0;
-        for (ChannelHandler h: handlers) {
-            if (h == null) {
-                break;
-            }
-            nHandlers ++;
-        }
-
-        if (nHandlers == 0) {
-            throw new IllegalArgumentException("handlers is empty.");
-        }
-
         ChannelPipeline p = pipeline();
         p.addLast(new ChannelInitializer<Channel>() {
             @Override

transport/src/test/java/io/netty/channel/embedded/EmbeddedChannelTest.java

@@ -122,4 +122,16 @@ public class EmbeddedChannelTest {
             throw cause;
         }
     }
+
+    @Test
+    public void testConstructWithOutHandler() {
+        EmbeddedChannel channel = new EmbeddedChannel();
+        Assert.assertTrue(channel.writeInbound(1));
+        Assert.assertTrue(channel.writeOutbound(2));
+        Assert.assertTrue(channel.finish());
+        Assert.assertSame(1, channel.readInbound());
+        Assert.assertNull(channel.readInbound());
+        Assert.assertSame(2, channel.readOutbound());
+        Assert.assertNull(channel.readOutbound());
+    }
 }