fix(dataset): import/export jinja template bug

[crabulous]

SUMMARY

While deleting the existing jinja parameter, in dataset editor, an empty string was saved instead of the None value, which was incorrectly processed during export/import. For bug fix, the @pre_load method was added, which would replace an empty string in template_params with the value None

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

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

[crabulous]

@supersetbot orglabel

[github-actions]

Congrats on making your first PR and thank you for contributing to Superset! 🎉 ❤️

We hope to see you in our Slack community too! Not signed up? Use our Slack App to self-register.

[john-bodley]

@crabulous thanks for the PR. Would you mind adding unit tests to prevent future regressions?

[crabulous]

@crabulous thanks for the PR. Would you mind adding unit tests to prevent future regressions?

I did it) Check my test, please!