[DUBBO-3713]: org.apache.dubbo.rpc.support.MockInvoker#getInterface should not return null

[beiwei30]

What is the purpose of the change

fix issue #3713

Brief changelog

XXXXX

Verifying this change

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[codecov-io]

Codecov Report

Merging #3716 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #3716      +/-   ##
============================================
+ Coverage     63.55%   63.57%   +0.02%     
  Complexity       71       71              
============================================
  Files           703      703              
  Lines         31001    31002       +1     
  Branches       5022     5022              
============================================
+ Hits          19702    19711       +9     
+ Misses         9027     9019       -8     
  Partials       2272     2272

Impacted Files	Coverage Δ	Complexity Δ
...pc/cluster/support/wrapper/MockClusterInvoker.java	73.21% <100%> (ø)	0 <0> (ø)	⬇️
...ava/org/apache/dubbo/rpc/support/MockProtocol.java	50% <100%> (ø)	0 <0> (ø)	⬇️
...java/org/apache/dubbo/rpc/support/MockInvoker.java	75% <100%> (+0.23%)	0 <0> (ø)	⬇️
...dubbo/remoting/exchange/support/DefaultFuture.java	71.42% <0%> (-2.05%)	0% <0%> (ø)
...he/dubbo/registry/multicast/MulticastRegistry.java	67.87% <0%> (-1.81%)	0% <0%> (ø)
.../dubbo/remoting/transport/netty4/NettyChannel.java	65.88% <0%> (+1.17%)	0% <0%> (ø)	⬇️
...exchange/support/header/HeaderExchangeHandler.java	62.79% <0%> (+2.32%)	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)	⬇️
... and 2 more

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[Moriadry-zz]

The construction should be the same with org.apache.dubbo.rpc.cluster.router.MockInvoker.class

[cvictory]

LGTM,
getInterface returning null is incorrect and add interface parameter in MockInvoker to fix the issue.