follow up for #3376 (#3377)

Author: beiwei30

dubbo-rpc/dubbo-rpc-api/src/main/java/org/apache/dubbo/rpc/protocol/AbstractInvoker.java

@@ -22,6 +22,7 @@
 import org.apache.dubbo.common.logger.Logger;
 import org.apache.dubbo.common.logger.LoggerFactory;
 import org.apache.dubbo.common.utils.ArrayUtils;
+import org.apache.dubbo.common.utils.CollectionUtils;
 import org.apache.dubbo.common.utils.NetUtils;
 import org.apache.dubbo.rpc.Invocation;
 import org.apache.dubbo.rpc.Invoker;
@@ -134,11 +135,11 @@ public Result invoke(Invocation inv) throws RpcException {
         }
         RpcInvocation invocation = (RpcInvocation) inv;
         invocation.setInvoker(this);
-        if (attachment != null && !attachment.isEmpty()) {
+        if (CollectionUtils.isNotEmptyMap(attachment)) {
             invocation.addAttachmentsIfAbsent(attachment);
         }
         Map<String, String> contextAttachments = RpcContext.getContext().getAttachments();
-        if (contextAttachments != null && !contextAttachments.isEmpty()) {
+        if (CollectionUtils.isNotEmptyMap(contextAttachments)) {
             /**
              * invocation.addAttachmentsIfAbsent(context){@link RpcInvocation#addAttachmentsIfAbsent(Map)}should not be used here,
              * because the {@link RpcContext#setAttachment(String, String)} is passed in the Filter when the call is triggered