[SCB-2689] fix rate limiting configuration changed cause NPE problem

[liubao68]

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[codecov-commenter]

Codecov Report

Merging #3349 (cca29c1) into master (618f4b7) will decrease coverage by 0.00%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master    #3349      +/-   ##
============================================
- Coverage     77.60%   77.59%   -0.01%     
  Complexity     1452     1452              
============================================
  Files          1648     1648              
  Lines         43720    43718       -2     
  Branches       3679     3678       -1     
============================================
- Hits          33928    33925       -3     
- Misses         8261     8262       +1     
  Partials       1531     1531              

Impacted Files	Coverage Δ
...g/apache/servicecomb/qps/QpsControllerManager.java	78.78% <81.81%> (-0.16%)	⬇️
...servicecomb/qps/ProviderQpsFlowControlHandler.java	87.50% <100.00%> (-0.74%)	⬇️
...in/java/org/apache/servicecomb/core/SCBEngine.java	79.28% <0.00%> (-1.20%)	⬇️
...mb/config/ConfigCenterConfigurationSourceImpl.java	10.63% <0.00%> (+1.06%)	⬆️
...ache/servicecomb/foundation/common/net/IpPort.java	86.66% <0.00%> (+3.33%)	⬆️

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