Simplify publishURL parsing in cmd

[Geod24]

Pretty simple, instead of having two separated code path, we only branch out if there is no value.

[codecov-io]

Codecov Report

Merging #924 into master will decrease coverage by 0.01%. The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #924      +/-   ##
==========================================
- Coverage   60.75%   60.74%   -0.02%     
==========================================
  Files         160      160              
  Lines       11250    11247       -3     
==========================================
- Hits         6835     6832       -3     
  Misses       3609     3609              
  Partials      806      806

Flag	Coverage Δ
#integration_tests_long_term	44.05% <71.42%> (+0.01%)	:arrow_up:
#integration_tests_node	40.55% <71.42%> (+0.01%)	:arrow_up:
#integration_tests_retry	37.73% <71.42%> (+0.02%)	:arrow_up:
#unittests	47.38% <71.42%> (+0.01%)	:arrow_up:

Impacted Files	Coverage Δ
cmd/sebak/cmd/run.go	56.7% <71.42%> (-0.23%)	:arrow_down:

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[Geod24]

Ping @Charleslee522 @spikeekips