Fix method may be null in RpcUtils,avoid throwing NullPointerException(#7626)

[skyguard1]

What is the purpose of the change

In this issue(#7626 the getReturnType method of org.apache.dubbo.rpc.support.RpcUtils, the method may be null and a NullPointerException will be thrown,this pr is to fix this issue, to avoid throwing NullPointerException in RpcUtils, thanks

Brief changelog

Verifying this change

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[codecov-commenter]

Codecov Report

Merging #7627 (bf414ff) into master (f8e25ec) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #7627      +/-   ##
============================================
- Coverage     59.11%   59.10%   -0.02%     
  Complexity      528      528              
============================================
  Files          1076     1076              
  Lines         43443    43443              
  Branches       6346     6347       +1     
============================================
- Hits          25681    25676       -5     
- Misses        14917    14938      +21     
+ Partials       2845     2829      -16     

Impacted Files	Coverage Δ	Complexity Δ
...in/java/org/apache/dubbo/rpc/support/RpcUtils.java	52.54% <100.00%> (-1.70%)	0.00 <0.00> (ø)
...in/java/org/apache/dubbo/common/utils/JVMUtil.java	81.13% <0.00%> (-11.33%)	0.00% <0.00%> (ø%)
...che/dubbo/remoting/transport/mina/MinaChannel.java	35.52% <0.00%> (-10.53%)	14.00% <0.00%> (-1.00%)
...o/remoting/transport/ChannelHandlerDispatcher.java	60.00% <0.00%> (-10.00%)	0.00% <0.00%> (ø%)
...mmon/serialize/kryo/utils/AbstractKryoFactory.java	75.43% <0.00%> (-3.51%)	0.00% <0.00%> (ø%)
...e/dubbo/registry/integration/RegistryProtocol.java	57.94% <0.00%> (-2.81%)	0.00% <0.00%> (ø%)
...ting/exchange/support/header/HeartbeatHandler.java	90.69% <0.00%> (-2.33%)	0.00% <0.00%> (ø%)
.../dubbo/remoting/transport/netty4/NettyChannel.java	59.40% <0.00%> (-1.99%)	0.00% <0.00%> (ø%)
...e/dubbo/remoting/transport/netty4/NettyClient.java	69.56% <0.00%> (-1.45%)	0.00% <0.00%> (ø%)
... and 18 more

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[AlbumenJ]

LGTM.