Fix tabular with no categorical fields

[ethanwharris]

What does this PR do?

Fixes #1143

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[codecov]

Codecov Report

Merging #1144 (0354b20) into master (700a9c8) will increase coverage by 6.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1144      +/-   ##
==========================================
+ Coverage   82.94%   89.18%   +6.23%     
==========================================
  Files         285      285              
  Lines       13008    13008              
==========================================
+ Hits        10790    11601     +811     
+ Misses       2218     1407     -811     

Flag	Coverage Δ
unittests	89.18% <100.00%> (+6.23%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/tabular/input.py	92.59% <100.00%> (ø)
flash/image/classification/data.py	97.63% <0.00%> (+1.57%)	⬆️
flash/core/data/transforms.py	96.49% <0.00%> (+1.75%)	⬆️
...ash/image/classification/integrations/baal/data.py	38.00% <0.00%> (+2.00%)	⬆️
flash/image/classification/input_transform.py	83.33% <0.00%> (+3.33%)	⬆️
flash/image/segmentation/data.py	100.00% <0.00%> (+3.70%)	⬆️
flash/image/classification/model.py	78.84% <0.00%> (+3.84%)	⬆️
.../image/classification/integrations/baal/dropout.py	42.85% <0.00%> (+4.76%)	⬆️
flash/image/classification/input.py	100.00% <0.00%> (+8.53%)	⬆️
flash/core/integrations/fiftyone/utils.py	64.70% <0.00%> (+13.72%)	⬆️
... and 35 more

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