v2: fixes to validation and upconversion

[dweindl]

• don't create unnecessary experiment tables
• write yaml file after updating the config dict
• check for missing experiments
• handle anonymous experiments

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.89%. Comparing base (6e762c6) to head (6053934).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
petab/v2/lint.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #351      +/-   ##
===========================================
+ Coverage    74.84%   74.89%   +0.04%     
===========================================
  Files           56       56              
  Lines         5597     5604       +7     
  Branches       980      982       +2     
===========================================
+ Hits          4189     4197       +8     
  Misses        1033     1033              
+ Partials       375      374       -1     

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[dilpath]

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