fix hystrix fallback bug

[yu199195]

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[codecov-commenter]

Codecov Report

Merging #4593 (e40196a) into master (1b961cd) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master    #4593   +/-   ##
=========================================
  Coverage     67.64%   67.65%           
- Complexity     7872     7875    +3     
=========================================
  Files          1077     1077           
  Lines         30767    30767           
  Branches       2805     2806    +1     
=========================================
+ Hits          20811    20814    +3     
+ Misses         8329     8325    -4     
- Partials       1627     1628    +1     

Impacted Files	Coverage Δ
...e/shenyu/plugin/base/fallback/FallbackHandler.java	36.84% <0.00%> (-17.01%)	⬇️
.../apache/shenyu/plugin/hystrix/command/Command.java	7.69% <0.00%> (+2.69%)	⬆️
.../shenyu/plugin/hystrix/command/HystrixCommand.java	55.55% <0.00%> (ø)
...plugin/hystrix/command/HystrixCommandOnThread.java	61.11% <0.00%> (ø)
...shenyu/web/fallback/DefaultFallbackController.java	75.00% <0.00%> (-25.00%)	⬇️

... and 2 files with indirect coverage changes

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