Bugfix/panic calculate ipset

[River-sh]

fix the bug of panic if cidr is empty

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 35.63%. Comparing base (d70eb16) to head (fa158b9).
Report is 1 commits behind head on main.

Files	Patch %	Lines
pkg/networkengine/routedriver/vxlan/vxlan.go	25.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #167      +/-   ##
==========================================
- Coverage   35.75%   35.63%   -0.12%     
==========================================
  Files          12       12              
  Lines        1474     1476       +2     
==========================================
- Hits          527      526       -1     
- Misses        840      842       +2     
- Partials      107      108       +1     

Flag	Coverage Δ
unittests	35.63% <25.00%> (-0.12%)	⬇️

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