- [ ] 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!!**


@Test
    public void testInvoke() {
        ServiceClassHolder.getInstance().pushServiceClass(DemoService.class);
        Exporter<IDemoService> exporter = protocol.export(proxy.getInvoker(new DemoService(), IDemoService.class, exportUrl));
        RpcInvocation rpcInvocation = new RpcInvocation("hello", new Class[]{Integer.class, Integer.class}, new Integer[]{2, 3});
        Result result = exporter.getInvoker().invoke(rpcInvocation);
        assertThat(result.getValue(), CoreMatchers.<Object>is(5));
    }
    @Test
    public void testFilter() {
        ServiceClassHolder.getInstance().pushServiceClass(DemoService.class);
        URL nettyUrl = exportUrl.addParameter(Constants.SERVER_KEY, "netty")
                .addParameter(Constants.EXTENSION_KEY, "org.apache.dubbo.rpc.protocol.rest.support.LoggingFilter");
        Exporter<IDemoService> exporter = protocol.export(proxy.getInvoker(new DemoService(), IDemoService.class, nettyUrl));
        IDemoService demoService = this.proxy.getProxy(protocol.refer(IDemoService.class, nettyUrl));
        Integer result = demoService.hello(1, 2);
        assertThat(result, is(3));
        exporter.unexport();
    }
    @Test(expected = RuntimeException.class)
    public void testRegFail() {
        ServiceClassHolder.getInstance().pushServiceClass(DemoService.class);
        URL nettyUrl = exportUrl.addParameter(Constants.EXTENSION_KEY, "com.not.existing.Filter");
        protocol.export(proxy.getInvoker(new DemoService(), IDemoService.class, nettyUrl));
    }
    @Test
    public void testDefaultPort() {
        assertThat(protocol.getDefaultPort(), is(80));
    }
}