fix ShenyuTrie NPE

[lan-dian]

fix #4530 This is my first pr and I will continue to contribute

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.

[lan-dian]

@moremind Could you please help me advance the test process? Thank you.

[codecov-commenter]

Codecov Report

Merging #4533 (14977d6) into master (df6d9c7) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #4533      +/-   ##
============================================
- Coverage     68.53%   68.52%   -0.01%     
  Complexity     7632     7632              
============================================
  Files          1028     1028              
  Lines         29441    29443       +2     
  Branches       2666     2667       +1     
============================================
- Hits          20178    20177       -1     
- Misses         7666     7667       +1     
- Partials       1597     1599       +2     

Impacted Files	Coverage Δ
...org/apache/shenyu/plugin/base/trie/ShenyuTrie.java	69.02% <0.00%> (-0.76%)	⬇️

... and 1 file with indirect coverage changes

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[lan-dian]

@moremind Hello, I have passed the unit test. Could you please help me review it? Thank you. 😄