bug: NPE in SpringStatManager

[XenoAmess]

No description provided.

[codecov-commenter]

Codecov Report

Base: 58.94% // Head: 58.94% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (5974073) compared to base (d92126a).
Patch coverage: 50.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #4525   +/-   ##
=========================================
  Coverage     58.94%   58.94%           
+ Complexity    26910    26909    -1     
=========================================
  Files          1268     1268           
  Lines        122755   122756    +1     
  Branches      23123    23124    +1     
=========================================
+ Hits          72355    72356    +1     
+ Misses        41381    41380    -1     
- Partials       9019     9020    +1     

Impacted Files	Coverage Δ
...a/druid/support/spring/stat/SpringStatManager.java	57.50% <50.00%> (-1.48%)	⬇️
.../java/com/alibaba/druid/util/JdbcSqlStatUtils.java	58.62% <0.00%> (-3.45%)	⬇️
...n/java/com/alibaba/druid/pool/DruidDataSource.java	71.98% <0.00%> (ø)
...baba/druid/pool/DruidDataSourceStatLoggerImpl.java	51.72% <0.00%> (+1.72%)	⬆️

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[lizongbo]

这里目前实际add的stat只有SpringStat类型的，不会触发抛异常的场景，但是先前代码写得绕，所以ide可能会提示该变量可能为null