[#2454] Correctly return null when DefaultChannelPipeline.firstContext() is called on empty pipeline

Motivation:
DefaultChannelPipeline.firstContext() should return null when the ipeline is empty. This is not the case atm.

Modification:
Fix incorrect check in DefaultChannelPipeline.firstContext() and add unit tests.

Result:
Correctly return null when DefaultChannelPipeline.firstContext() is called on empty pipeline.

transport/src/main/java/io/netty/channel/DefaultChannelPipeline.java

@@ -667,7 +667,7 @@ final class DefaultChannelPipeline implements ChannelPipeline {
     @Override
     public ChannelHandlerContext firstContext() {
         DefaultChannelHandlerContext first = head.next;
-        if (first == head) {
+        if (first == tail) {
             return null;
         }
         return head.next;

transport/src/test/java/io/netty/channel/DefaultChannelPipelineTest.java

@@ -524,6 +524,30 @@ public class DefaultChannelPipelineTest {
         assertEquals(0, buffer.refCnt());
     }
 
+    @Test
+    public void testFirstContextEmptyPipeline() throws Exception {
+        ChannelPipeline pipeline = new LocalChannel().pipeline();
+        assertNull(pipeline.firstContext());
+    }
+
+    @Test
+    public void testLastContextEmptyPipeline() throws Exception {
+        ChannelPipeline pipeline = new LocalChannel().pipeline();
+        assertNull(pipeline.lastContext());
+    }
+
+    @Test
+    public void testFirstHandlerEmptyPipeline() throws Exception {
+        ChannelPipeline pipeline = new LocalChannel().pipeline();
+        assertNull(pipeline.first());
+    }
+
+    @Test
+    public void testLastHandlerEmptyPipeline() throws Exception {
+        ChannelPipeline pipeline = new LocalChannel().pipeline();
+        assertNull(pipeline.last());
+    }
+
     private static int next(DefaultChannelHandlerContext ctx) {
         DefaultChannelHandlerContext next = ctx.next;
         if (next == null) {