p2p: fix empty allow list

[corverroos]

When --p2p-allowlist is not defined, it should default to "allow all". But an empty string results in a empty netlist which results in "allow none". This is only now a problem since geth's discv5 previously ignored NetRestrict.

category: bug
ticket: none

[corverroos]

See ethereum/go-ethereum#25304.
This should result in these tests passing: #868

[codecov]

Codecov Report

Merging #872 (dc3be62) into main (827d675) will increase coverage by 0.05%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##             main     #872      +/-   ##
==========================================
+ Coverage   54.39%   54.45%   +0.05%     
==========================================
  Files         114      114              
  Lines       12211    12213       +2     
==========================================
+ Hits         6642     6650       +8     
+ Misses       4603     4598       -5     
+ Partials      966      965       -1     

Impacted Files	Coverage Δ
p2p/discovery.go	47.88% <28.57%> (-1.39%)	⬇️
core/dutydb/memory.go	70.63% <0.00%> (-1.28%)	⬇️
core/qbft/qbft.go	81.54% <0.00%> (+0.42%)	⬆️
core/leadercast/transport.go	76.33% <0.00%> (+1.18%)	⬆️
dkg/transport.go	66.66% <0.00%> (+12.96%)	⬆️

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