- [ ] 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!!**


import org.apache.dubbo.common.Constants;
import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.extension.ExtensionLoader;
import org.apache.dubbo.common.utils.NetUtils;
import org.apache.dubbo.rpc.Exporter;
import org.apache.dubbo.rpc.Protocol;
import org.apache.dubbo.rpc.ProxyFactory;
import org.apache.dubbo.rpc.Result;
import org.apache.dubbo.rpc.RpcContext;
import org.apache.dubbo.rpc.RpcException;
import org.apache.dubbo.rpc.RpcInvocation;
import org.apache.dubbo.rpc.ServiceClassHolder;
import org.hamcrest.CoreMatchers;
import org.junit.After;
import org.junit.Test;
import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;
public class RestProtocolTest {
    private Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getExtension("rest");
    private ProxyFactory proxy = ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();
    private final int availablePort = NetUtils.getAvailablePort();
    private final URL exportUrl = URL.valueOf("rest://127.0.0.1:" + availablePort + "/rest");
    @After
    public void tearDown() {
        protocol.destroy();
    }
    @Test
    public void testExport() {
        ServiceClassHolder.getInstance().pushServiceClass(DemoService.class);
        RpcContext.getContext().setAttachment("timeout", "200");