[Dubbo-4299]Fix NPE when PojoUtils realize Null element in collection(#4299)

[HzjNeverStop]

What is the purpose of the change

https://github.com/apache/dubbo/issues/4299

Brief changelog

Fix NPE when PojoUtils realize Null element in collection

Verifying this change

XXXXX

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[HzjNeverStop]

reviewed

[zonghaishang]

Please provide unit tests

[HzjNeverStop]

Please provide unit tests

done

[codecov-io]

Codecov Report

Merging #4300 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #4300      +/-   ##
============================================
- Coverage     62.94%   62.92%   -0.02%     
+ Complexity      506      504       -2     
============================================
  Files           769      769              
  Lines         33041    33041              
  Branches       5215     5216       +1     
============================================
- Hits          20798    20792       -6     
- Misses         9850     9853       +3     
- Partials       2393     2396       +3

Impacted Files	Coverage Δ	Complexity Δ
.../java/org/apache/dubbo/common/utils/PojoUtils.java	72.07% <100%> (+0.56%)	0 <0> (ø)	⬇️
.../apache/dubbo/qos/protocol/QosProtocolWrapper.java	64.1% <0%> (-17.95%)	0% <0%> (ø)
...apache/dubbo/common/config/ConfigurationUtils.java	64% <0%> (-8%)	0% <0%> (ø)
...ng/exchange/support/header/HeartbeatTimerTask.java	73.68% <0%> (-5.27%)	0% <0%> (ø)
.../apache/dubbo/remoting/transport/AbstractPeer.java	63.04% <0%> (-2.18%)	0% <0%> (ø)
...ache/dubbo/registry/multiple/MultipleRegistry.java	62.67% <0%> (-1.41%)	20% <0%> (-2%)
...pache/dubbo/remoting/transport/AbstractServer.java	47.91% <0%> (-1.05%)	0% <0%> (ø)
.../exchange/support/header/HeaderExchangeServer.java	66.98% <0%> (-0.95%)	0% <0%> (ø)
...g/apache/dubbo/registry/consul/ConsulRegistry.java	59.6% <0%> (-0.67%)	27% <0%> (ø)
... and 3 more

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