add checks for TaskDoc's model_post_init

[tschaume]

@tsmathis @esoteric-ephemera fix for these two forum threads:

• https://matsci.org/t/mp-api-unavailable-for-charge-density-query-in-jun-2024/56058/5
• https://matsci.org/t/how-can-i-download-the-chgcar-file-for-multiples-mp-ids-and-convert-them-into-individual-npy-files/56365/7

When the task model is initiated from a database document, the document might not contain the necessary fields depending on what the user requested.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.00%. Comparing base (f3d65de) to head (47cefe5).
Report is 2 commits behind head on main.

Files	Patch %	Lines
emmet-core/emmet/core/tasks.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1074      +/-   ##
==========================================
- Coverage   90.00%   90.00%   -0.01%     
==========================================
  Files         143      143              
  Lines       13432    13438       +6     
==========================================
+ Hits        12090    12095       +5     
- Misses       1342     1343       +1     

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