- [ ] 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!!**


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package org.apache.dubbo.rpc.protocol.rest;