Fix NullPointerException when MCP is disabled

Add null check for mcpRequestHandler before accessing getMcpEndpoint() method. This prevents NullPointerException when arthas.mcpEndpoint is set to empty string (MCP disabled) and accessing endpoints like /arthas-output/.

Co-authored-by: hengyunabc <1683936+hengyunabc@users.noreply.github.com>

Author: Copilot

core/src/main/java/com/taobao/arthas/core/shell/term/impl/http/HttpRequestHandler.java

@@ -76,11 +76,13 @@ protected void channelRead0(ChannelHandlerContext ctx, FullHttpRequest request)
                 }
 
                 //handle mcp request
-                String mcpEndpoint = mcpRequestHandler.getMcpEndpoint();
-                if (mcpEndpoint.equals(path)) {
-                    mcpRequestHandler.handle(ctx, request);
-                    isMcpHandled = true;
-                    return;
+                if (mcpRequestHandler != null) {
+                    String mcpEndpoint = mcpRequestHandler.getMcpEndpoint();
+                    if (mcpEndpoint.equals(path)) {
+                        mcpRequestHandler.handle(ctx, request);
+                        isMcpHandled = true;
+                        return;
+                    }
                 }
 
                 //handle webui requests