Backport PR #826 on branch 1.x (Defer preferred_dir validation until root_dir is set)

[meeseeksmachine]

Backport PR #826: Defer preferred_dir validation until root_dir is set

[codecov-commenter]

Codecov Report

Merging #828 (6b219cf) into 1.x (2e31f90) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              1.x     #828      +/-   ##
==========================================
- Coverage   70.47%   70.45%   -0.03%     
==========================================
  Files          62       62              
  Lines        7540     7554      +14     
  Branches     1250     1255       +5     
==========================================
+ Hits         5314     5322       +8     
- Misses       1843     1851       +8     
+ Partials      383      381       -2     

Impacted Files	Coverage Δ
jupyter_server/pytest_plugin.py	88.42% <100.00%> (+0.10%)	⬆️
jupyter_server/serverapp.py	65.68% <100.00%> (+0.38%)	⬆️
jupyter_server/services/kernels/kernelmanager.py	78.64% <0.00%> (-1.95%)	⬇️

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