Fix crash when discovery fails while using -p no:terminal

[asottile]

Resolves #5505

[nicoddemus]

Thanks for the quick fix!

[codecov]

Codecov Report

Merging #5506 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5506      +/-   ##
==========================================
+ Coverage   96.09%   96.09%   +<.01%     
==========================================
  Files         117      117              
  Lines       25643    25647       +4     
  Branches     2483     2483              
==========================================
+ Hits        24642    24646       +4     
  Misses        696      696              
  Partials      305      305

Impacted Files	Coverage Δ
src/_pytest/nodes.py	95.5% <100%> (ø)	⬆️
testing/test_config.py	99.83% <100%> (ø)	⬆️

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