fix: Issue when delete all tags on import [FC-0036]

[ChrisChV]

Description

This fix an error when it comes to performing actions on a taxonomy with tags that have external_id=None

More info

Part of: https://github.com/openedx/modular-learning/issues/126

Testing instructions

Please ensure that the tests cover the expected behavior

After merge

• [ ] Create a tag to upload the new version

[openedx-webhooks]

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[ChrisChV]

@rpenido It's ready for another review

[rpenido]

LGTM @ChrisChV! :+1:

• [x] I read through the code
• [x] I checkout the branch and run the tests
• [x] I imported tags over a Taxonomy that originated this bug

@ormsbee If you have time, could you please review/merge this PR while @bradenmacdonald is off?

Thank you!

[ormsbee]

@rpenido: I'm also technically off until Jan 2. Is this blocking your work?

[rpenido]

@rpenido: I'm also technically off until Jan 2. Is this blocking your work?

No @ormsbee. This is not urgent and could wait. Sorry to bother you!

Thank you for your response and have a good holiday! :smiley:

[ChrisChV]

@bradenmacdonald It's ready for another review

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