Skip docker-user ipv6 manipulations if the docker-user chain is not found

[hellt]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 57.69231% with 22 lines in your changes missing coverage. Please review.

Project coverage is 52.52%. Comparing base (6ac0c67) to head (e6afc82).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
runtime/docker/firewall/nftables/client.go	57.69%	16 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2410      +/-   ##
==========================================
- Coverage   52.67%   52.52%   -0.16%     
==========================================
  Files         168      168              
  Lines       17039    17052      +13     
==========================================
- Hits         8976     8956      -20     
- Misses       7143     7167      +24     
- Partials      920      929       +9     

Files with missing lines	Coverage Δ
runtime/docker/firewall/nftables/client.go	68.83% <57.69%> (-10.53%)	⬇️

... and 3 files with indirect coverage changes