- [ ] I have searched the [issues](https://github.com/apache/incubator-dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [FAQ](https://github.com/apache/incubator-dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.6.1 - newest

### Step to reproduce this issue

1.  
String value = null;
RpcContext.getContext().getAttachments().put("demokey", value)
2. 
then invoke one method will NullPointException

**The Exception reason:
In RpcContextFilter.java，public void filter(ClientRequestContext requestContext) throws IOException
for (Map.Entry<String, String> entry : RpcContext.getContext().getAttachments().entrySet()) {
            if (entry.getValue().contains(",") || entry.getValue().contains("=")

the entry.getValue not judge null!!**


Exporter<IDemoService> exporter = protocol.export(proxy.getInvoker(new DemoService(), IDemoService.class, exportUrl));
        IDemoService demoService = this.proxy.getProxy(protocol.refer(IDemoService.class, exportUrl));
        Integer echoString = demoService.hello(1, 2);
        assertThat(echoString, is(3));
        exporter.unexport();
    }
    @Test
    public void testNettyServer() {
        ServiceClassHolder.getInstance().pushServiceClass(DemoService.class);
        URL nettyUrl = exportUrl.addParameter(Constants.SERVER_KEY, "netty");
        Exporter<IDemoService> exporter = protocol.export(proxy.getInvoker(new DemoService(), IDemoService.class, nettyUrl));
        IDemoService demoService = this.proxy.getProxy(protocol.refer(IDemoService.class, nettyUrl));
        Integer echoString = demoService.hello(10, 10);
        assertThat(echoString, is(20));
        exporter.unexport();
    }
    @Test(expected = RpcException.class)
    public void testServletWithoutWebConfig() {
        ServiceClassHolder.getInstance().pushServiceClass(DemoService.class);
        URL servletUrl = exportUrl.addParameter(Constants.SERVER_KEY, "servlet");
        protocol.export(proxy.getInvoker(new DemoService(), IDemoService.class, servletUrl));
    }
    @Test(expected = RpcException.class)
    public void testErrorHandler() {
        ServiceClassHolder.getInstance().pushServiceClass(DemoService.class);
        URL nettyUrl = exportUrl.addParameter(Constants.SERVER_KEY, "netty");
        Exporter<IDemoService> exporter = protocol.export(proxy.getInvoker(new DemoService(), IDemoService.class, nettyUrl));
        IDemoService demoService = this.proxy.getProxy(protocol.refer(IDemoService.class, nettyUrl));
        demoService.error();
    }