Fixed the extreme strain on the world tick thread.

Block changes were sent even with empty changesets, causing a lot of lost CPU cycles.

source/Chunk.cpp

@@ -573,6 +573,11 @@ void cChunk::ProcessQueuedSetBlocks(void)
 
 void cChunk::BroadcastPendingBlockChanges(void)
 {
+	if (m_PendingSendBlocks.empty())
+	{
+		return;
+	}
+	
 	for (cClientHandleList::iterator itr = m_LoadedByClient.begin(), end = m_LoadedByClient.end(); itr != end; ++itr)
 	{
 		(*itr)->SendBlockChanges(m_PosX, m_PosZ, m_PendingSendBlocks);

source/ClientHandle.cpp

@@ -1502,6 +1502,8 @@ void cClientHandle::SendBlockChange(int a_BlockX, int a_BlockY, int a_BlockZ, BL
 
 void cClientHandle::SendBlockChanges(int a_ChunkX, int a_ChunkZ, const sSetBlockVector & a_Changes)
 {
+	ASSERT(!a_Changes.empty());  // We don't want to be sending empty change packets!
+	
 	m_Protocol->SendBlockChanges(a_ChunkX, a_ChunkZ, a_Changes);
 }