Fix(#1974):fix shutdown InternalSignal default value

[zhaoyunxing92]

What this PR does:

#1974 fix shotdown InternalSignal default value

Which issue(s) this PR fixes:
Fixes #

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[AlexStocks]

不需要加锁保护下吗？

[zhaoyunxing92]

应该不需要，没有并发的场景

[AlexStocks]

pls fix the file confliction

[codecov-commenter]

Codecov Report

Merging #2003 (aab693f) into 3.0 (013f0b2) will increase coverage by 0.21%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##              3.0    #2003      +/-   ##
==========================================
+ Coverage   44.38%   44.59%   +0.21%     
==========================================
  Files         283      283              
  Lines       17067    17073       +6     
==========================================
+ Hits         7575     7614      +39     
+ Misses       8690     8653      -37     
- Partials      802      806       +4     

Impacted Files	Coverage Δ
config/graceful_shutdown.go	3.88% <0.00%> (ø)
config/graceful_shutdown_config.go	89.83% <66.66%> (-2.90%)	⬇️
cluster/cluster/available/cluster_invoker.go	66.66% <0.00%> (-6.67%)	⬇️
common/url.go	59.04% <0.00%> (-0.27%)	⬇️
cluster/router/tag/router.go	59.64% <0.00%> (ø)
cluster/cluster/zoneaware/cluster_interceptor.go	40.00% <0.00%> (ø)
cluster/cluster/base/cluster_invoker.go	37.77% <0.00%> (+13.33%)	⬆️
registry/zookeeper/service_discovery.go	17.07% <0.00%> (+15.85%)	⬆️

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[justxuewei]

typo: shotdown -> shutdown