[1.12] Fixes Exception when getting modlist for a vanilla client (#4028)

src/main/java/net/minecraftforge/fml/common/network/handshake/NetworkDispatcher.java

@@ -113,7 +113,7 @@ public class NetworkDispatcher extends SimpleChannelInboundHandler<Packet<?>> im
     private final EmbeddedChannel handshakeChannel;
     private NetHandlerPlayServer serverHandler;
     private INetHandler netHandler;
-    private Map<String,String> modList;
+    private Map<String,String> modList = Collections.emptyMap();
     private int overrideLoginDim;
 
     public NetworkDispatcher(NetworkManager manager)