fix(issue-7065): append condition to STI child entity join

[yohannpoli]

Description of change

Add discriminator condition on JOIN statement for child entity of STI.

Fixes #7065
Fixes #3181

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run lint passes with this change
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• [not needed] Documentation has been updated to reflect this change
• The new commits follow conventions explained in CONTRIBUTING.md

[pleerock]

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