[SCB-2813]DynamicProperties: when key removed should get default value

[liubao68]

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[codecov-commenter]

Codecov Report

Merging #3978 (dfd8ebc) into master (06b2b5f) will increase coverage by 0.20%.
The diff coverage is 6.97%.

❗ Current head dfd8ebc differs from pull request most recent head aa45d9b. Consider uploading reports for the commit aa45d9b to get more accurate results

@@             Coverage Diff              @@
##             master    #3978      +/-   ##
============================================
+ Coverage     43.80%   44.00%   +0.20%     
- Complexity     5311     5329      +18     
============================================
  Files          1391     1392       +1     
  Lines         34023    34041      +18     
  Branches       3336     3331       -5     
============================================
+ Hits          14903    14979      +76     
+ Misses        17929    17874      -55     
+ Partials       1191     1188       -3     

Files	Coverage Δ
.../servicecomb/config/ConfigurationChangedEvent.java	43.75% <ø> (ø)
...config/cc/ConfigCenterDynamicPropertiesSource.java	61.84% <0.00%> (-2.55%)	⬇️
...ervicecomb/config/kie/client/KieConfigManager.java	0.00% <0.00%> (ø)
...icecomb/config/kie/KieDynamicPropertiesSource.java	0.00% <0.00%> (ø)
...comb/config/center/client/ConfigCenterManager.java	62.06% <0.00%> (-4.60%)	⬇️
...onfig/kie/client/KieConfigurationChangedEvent.java	0.00% <0.00%> (ø)
...ache/servicecomb/config/DynamicPropertiesImpl.java	22.38% <20.68%> (+5.34%)	⬆️
.../client/ConfigCenterConfigurationChangedEvent.java	0.00% <0.00%> (ø)

... and 17 files with indirect coverage changes

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