fix(dataset-api): allow undefined schema in post

[villebro]

SUMMARY

The POST schema in the datasets API didn't accept None values, unlike the other schemas (check e.g. PUT here:

superset/superset/datasets/schemas.py

Line 95 in 50535e4

schema = fields.String(allow_none=True, validate=Length(0, 255))

)

BEFORE

Previously saving a dataset with an undefined schema caused an error:

TESTING INSTRUCTIONS

1. Go to SQL Lab, choose a db but don't select a schema
2. Write a query, create a chart, and try to save as a dataset
3. Get error

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #24242 (6752df4) into master (50535e4) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 6752df4 differs from pull request most recent head 2373881. Consider uploading reports for the commit 2373881 to get more accurate results

@@           Coverage Diff           @@
##           master   #24242   +/-   ##
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  Coverage   68.30%   68.30%           
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  Files        1957     1957           
  Lines       75594    75594           
  Branches     8224     8224           
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  Hits        51637    51637           
  Misses      21849    21849           
  Partials     2108     2108           

Flag	Coverage Δ
hive	53.36% <100.00%> (ø)
mysql	78.91% <100.00%> (ø)
postgres	78.99% <100.00%> (ø)
presto	53.29% <100.00%> (ø)
python	82.81% <100.00%> (ø)
sqlite	77.53% <100.00%> (ø)
unit	53.42% <100.00%> (ø)

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Impacted Files	Coverage Δ
superset/datasets/schemas.py	97.48% <100.00%> (ø)

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[villebro]

All the other tests have "", so I figured changing just one of these to None should be sufficient (no point in adding a full test for this)

[kgabryje]

LGTM