Allow tags to be None in validation

[munrojm]

Allow task documents to be validated with or without tags

[codecov-commenter]

Codecov Report

Merging #419 (113a6e9) into main (795ed51) will decrease coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
- Coverage   98.06%   97.87%   -0.19%     
==========================================
  Files         109       87      -22     
  Lines       23205    21461    -1744     
==========================================
- Hits        22755    21005    -1750     
- Misses        450      456       +6     

Impacted Files	Coverage Δ
emmet-core/emmet/core/vasp/task_valid.py	100.00% <100.00%> (ø)
emmet-core/emmet/core/utils.py	57.69% <0.00%> (-32.70%)	⬇️
emmet-core/emmet/core/oxidation_states.py	73.33% <0.00%> (-17.78%)	⬇️
emmet-core/emmet/core/qchem/molecule.py	80.18% <0.00%> (-16.99%)	⬇️
emmet-core/emmet/core/summary.py	80.29% <0.00%> (-10.22%)	⬇️
emmet-core/emmet/core/settings.py	88.13% <0.00%> (-8.48%)	⬇️
emmet-core/emmet/core/molecules/atomic.py	93.15% <0.00%> (-2.74%)	⬇️
emmet-core/emmet/core/mpid.py	94.44% <0.00%> (-1.86%)	⬇️
emmet-core/emmet/core/molecules/bonds.py	80.72% <0.00%> (-1.81%)	⬇️
emmet-core/emmet/core/electronic_structure.py	94.54% <0.00%> (-0.61%)	⬇️
... and 23 more

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