fix yaml empty list issue

[xusida]

What's the purpose of this PR

fix:
Include empty list configuration
yaml file causes exception (NPE) to be thrown

Which issue(s) this PR fixes:

Fixes #41

Brief changelog

XXXXX

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[codecov]

Codecov Report

Merging #42 (b63c0a8) into main (5344bc4) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main      #42      +/-   ##
============================================
- Coverage     68.59%   68.54%   -0.05%     
+ Complexity     1206     1205       -1     
============================================
  Files           171      171              
  Lines          5196     5198       +2     
  Branches        561      562       +1     
============================================
- Hits           3564     3563       -1     
- Misses         1371     1372       +1     
- Partials        261      263       +2     

Files	Coverage Δ
...m/ctrip/framework/apollo/util/yaml/YamlParser.java	94.36% <100.00%> (+0.16%)	⬆️

... and 2 files with indirect coverage changes