Fix bug with CacheFilter when use async call.

[carryxyh]

When use async call with cacheFilter, we will cache a empty result which is incorrect.

[carryxyh]

related comment here:
#3185 (comment)

[codecov-io]

Codecov Report

Merging #3731 into master will increase coverage by 0.05%.
The diff coverage is 54.16%.

@@             Coverage Diff              @@
##             master    #3731      +/-   ##
============================================
+ Coverage     63.58%   63.64%   +0.05%     
  Complexity       71       71              
============================================
  Files           703      703              
  Lines         31000    31010      +10     
  Branches       5022     5025       +3     
============================================
+ Hits          19712    19736      +24     
+ Misses         9018     9005      -13     
+ Partials       2270     2269       -1

Impacted Files	Coverage Δ	Complexity Δ
...ava/org/apache/dubbo/cache/filter/CacheFilter.java	64.51% <54.16%> (-11.68%)	0 <0> (ø)
.../java/org/apache/dubbo/config/ReferenceConfig.java	60.72% <0%> (-0.37%)	0% <0%> (ø)
...dubbo/remoting/exchange/support/DefaultFuture.java	73.46% <0%> (+0.68%)	0% <0%> (ø)	⬇️
...ache/dubbo/cache/support/expiring/ExpiringMap.java	42.1% <0%> (+1.75%)	0% <0%> (ø)	⬇️
...exchange/support/header/HeaderExchangeHandler.java	62.79% <0%> (+2.32%)	0% <0%> (ø)	⬇️
...bo/rpc/cluster/support/FailbackClusterInvoker.java	70.49% <0%> (+3.27%)	0% <0%> (ø)	⬇️
.../org/apache/dubbo/remoting/ExecutionException.java	21.05% <0%> (+5.26%)	0% <0%> (ø)	⬇️
...ng/exchange/support/header/HeartbeatTimerTask.java	78.94% <0%> (+5.26%)	0% <0%> (ø)	⬇️
...ng/transport/dispatcher/all/AllChannelHandler.java	57.14% <0%> (+5.71%)	0% <0%> (ø)	⬇️
.../remoting/transport/netty4/NettyServerHandler.java	71.42% <0%> (+9.52%)	0% <0%> (ø)	⬇️
... and 1 more

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[ralf0131]

Looks overall good to me. @chickenlj Could you please take a look as well?

[carryxyh]

Related to #3997