belongsToMany.add returns array of intermediate table instance

[u9r52sld]

…es table

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• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
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Description of change

belongsToMany.add returns array of intermediate table instance, not nested array of array.

See issue: #8377

[codecov]

Codecov Report

Merging #9700 into master will increase coverage by <.01%.
The diff coverage is 100%.