Fix unsetting language

[AlexVelezLl]

Summary

Description of the change(s) you made

This PR fixes the bug when removing the language from a node. Until now, when deleting a language it was set to undefined, so the Changes model did not detect it as a change. Changing the saved value to null forces the Changes model to know that this value has been removed.

Manual verification steps performed

1. Edited a Channel.
2. Set a language of a resource.
3. Unset the language of the resource.

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References

Fix #4493.

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[rtibbles]

Makes sense to me! Need the null value for Dexie to pick up the change event. I have not manually tested though.

[bjester]

Works for me!

[bjester]

@AlexVelezLl The one thing I noticed in your bulk editing branch was that there was no option to unset the language, i.e. use the default behavior of the folder's language. Perhaps this was an oversight in the original design. That lead me to 'can this be done currently', which you could but it was broken, hence the bug. I'm thinking for apples to apples comparison, it might be good to retain this ability in the bulk language editing modal?

[AlexVelezLl]

Hmm we can make the Radio Button clearable, so if a user clicks again on the selected radio button we unselect the language? Could this work? @bjester

[bjester]

@AlexVelezLl Yeah you might ask in #product or #design on slack what might be best. My thought was that a dedicated radio button for 'Use folder language' would make sense but that isn't my call.