fix(nodebuilder/p2p): fix autonat == nil panic in test

[renaynay]

Fixes a panic that can occur in tests if autonat is nil

Ref https://github.com/celestiaorg/celestia-node/actions/runs/13833242772/job/38702557881?pr=4149#step:4:449

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 44.86%. Comparing base (2469e7a) to head (dffd671).
Report is 462 commits behind head on main.

Files with missing lines	Patch %	Lines
nodebuilder/p2p/reachability.go	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4170      +/-   ##
==========================================
+ Coverage   44.83%   44.86%   +0.02%     
==========================================
  Files         265      310      +45     
  Lines       14620    22732    +8112     
==========================================
+ Hits         6555    10198    +3643     
- Misses       7313    11448    +4135     
- Partials      752     1086     +334     

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[Wondertan]

Ahh, I guess I didn't look at integration tests...
We need to fix them otheriwise I'll keep ignoring them 😂