[Dubbo-6960] fix bug: avoid npe when encode request

[wikiwikiwiki]

fix #6960 npe。
垃圾处理器非常擅长处理这种稍纵即逝的对象，没有必要做这个优化，反而导致了npe。

[codecov-io]

Codecov Report

Merging #6961 (f4b66c1) into master (3fc2d10) will decrease coverage by 0.37%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #6961      +/-   ##
============================================
- Coverage     59.44%   59.06%   -0.38%     
+ Complexity      511      505       -6     
============================================
  Files          1028     1028              
  Lines         41519    41526       +7     
  Branches       6037     6021      -16     
============================================
- Hits          24679    24528     -151     
- Misses        14104    14220     +116     
- Partials       2736     2778      +42     

Impacted Files	Coverage Δ	Complexity Δ
...va/org/apache/dubbo/remoting/exchange/Request.java	72.54% <0.00%> (-13.82%)	0.00 <0.00> (ø)
...dubbo/remoting/exchange/support/DefaultFuture.java	74.35% <0.00%> (-13.68%)	0.00 <0.00> (ø)
...rg/apache/dubbo/remoting/utils/PayloadDropper.java	0.00% <0.00%> (-46.16%)	0.00% <0.00%> (ø%)
...ng/transport/dispatcher/all/AllChannelHandler.java	62.06% <0.00%> (-27.59%)	0.00% <0.00%> (ø%)
...dubbo/common/status/support/LoadStatusChecker.java	46.15% <0.00%> (-15.39%)	0.00% <0.00%> (ø%)
...ng/transport/dispatcher/WrappedChannelHandler.java	47.82% <0.00%> (-15.22%)	0.00% <0.00%> (ø%)
...mmon/threadpool/support/AbortPolicyWithReport.java	85.00% <0.00%> (-12.50%)	0.00% <0.00%> (ø%)
...va/org/apache/dubbo/remoting/TimeoutException.java	22.22% <0.00%> (-11.12%)	0.00% <0.00%> (ø%)
...he/dubbo/common/threadpool/ThreadlessExecutor.java	52.17% <0.00%> (-10.87%)	0.00% <0.00%> (ø%)
... and 41 more

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[chickenlj]

LGTM

[AlbumenJ]

LGTM.