Fix tabular with embedding_sizes=None

[ethanwharris]

What does this PR do?

Fixes #1065

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[codecov]

Codecov Report

Merging #1067 (1b30f6d) into master (dd72360) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1067      +/-   ##
==========================================
- Coverage   87.58%   87.57%   -0.02%     
==========================================
  Files         280      280              
  Lines       12743    12747       +4     
==========================================
+ Hits        11161    11163       +2     
- Misses       1582     1584       +2     

Flag	Coverage Δ
unittests	87.57% <50.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
flash/tabular/regression/model.py	54.34% <0.00%> (-2.48%)	⬇️
flash/tabular/classification/model.py	93.87% <100.00%> (+0.26%)	⬆️

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