Don't panic if UrlExecCommand is not set during config

[synfinatic]

Never will support UrlExecCommand as a single string, so simply the code to be []string so we don't have to deal with interface{}

Fixes #385

[codecov]

Codecov Report

Merging #386 (df9e2f5) into main (71a97d1) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #386      +/-   ##
==========================================
+ Coverage   76.00%   76.09%   +0.09%     
==========================================
  Files          18       18              
  Lines        2279     2288       +9     
==========================================
+ Hits         1732     1741       +9     
  Misses        480      480              
  Partials       67       67              

Impacted Files	Coverage Δ
sso/awssso.go	65.96% <ø> (ø)
sso/settings.go	67.32% <ø> (ø)
internal/utils/url.go	100.00% <100.00%> (ø)

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