[Dubbo-3727] Fix NullPointerException when protocoluff protocol serializes an empty object

[fitzf]

What is the purpose of the change

Fixed the problem that the protocoluff protocol is not judged empty when serialized

Brief changelog

XXXXX

Verifying this change

XXXXX

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[codecov-io]

Codecov Report

Merging #3734 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master   #3734      +/-   ##
===========================================
+ Coverage     63.58%   63.6%   +0.01%     
  Complexity       71      71              
===========================================
  Files           703     703              
  Lines         31000   31000              
  Branches       5022    5022              
===========================================
+ Hits          19712   19716       +4     
- Misses         9018    9020       +2     
+ Partials       2270    2264       -6

Impacted Files	Coverage Δ	Complexity Δ
...n/serialize/protostuff/ProtostuffObjectOutput.java	100% <100%> (ø)	0 <0> (ø)	⬇️
...pache/dubbo/registry/support/AbstractRegistry.java	79.52% <0%> (-1.19%)	0% <0%> (ø)
...dubbo/remoting/exchange/support/DefaultFuture.java	74.14% <0%> (+1.36%)	0% <0%> (ø)	⬇️
...bo/rpc/cluster/support/FailbackClusterInvoker.java	70.49% <0%> (+3.27%)	0% <0%> (ø)	⬇️
.../apache/dubbo/remoting/transport/AbstractPeer.java	67.39% <0%> (+4.34%)	0% <0%> (ø)	⬇️
...ng/exchange/support/header/HeartbeatTimerTask.java	78.94% <0%> (+5.26%)	0% <0%> (ø)	⬇️

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[CrazyHZM]

LGTM

[fitzf]

@CrazyHZM Will this merge request be released in 2.7.1-RELEASE?

[CrazyHZM]

@CrazyHZM Will this merge request be released in 2.7.1-RELEASE?

This merge request will be released in 2.7.2.