- [ ] 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!!**


int index = header.indexOf("=");
                if (index > 0) {
                    String key = header.substring(0, index);
                    String value = header.substring(index + 1);
                    if (!StringUtils.isEmpty(key)) {
                        RpcContext.getContext().setAttachment(key.trim(), value.trim());
                    }
                }
            }
        }
    }
    @Override
    public void filter(ClientRequestContext requestContext) throws IOException {
        int size = 0;
        for (Map.Entry<String, String> entry : RpcContext.getContext().getAttachments().entrySet()) {
            if (entry.getValue().contains(",") || entry.getValue().contains("=")
                    || entry.getKey().contains(",") || entry.getKey().contains("=")) {
                throw new IllegalArgumentException("The attachments of " + RpcContext.class.getSimpleName() + " must not contain ',' or '=' when using rest protocol");
            }
            
            size += entry.getValue().getBytes("UTF-8").length;
            if (size > MAX_HEADER_SIZE) {
                throw new IllegalArgumentException("The attachments of " + RpcContext.class.getSimpleName() + " is too big");
            }
            String attachments = entry.getKey() + "=" + entry.getValue();
            requestContext.getHeaders().add(DUBBO_ATTACHMENT_HEADER, attachments);
        }
    }
}