record-communities: set branding is broken

[zzacharo]

Package version (if known): 12.0.0b3.dev5

Describe the bug

When a user tries to the set the branding of the record via the record details page/manage communities button, then the record.parent.communities.default is getting removed if it exists or doesn't get updated if was absent.

Steps to Reproduce

1. Create a public record and publish it individually.
2. Go the record details page.
3. Submit the record in a couple of communities.
4. Scroll to the "Communities" section, click on the gear icon and then "Manage communities" option.
5. Click on "Set branding" button on one of the not selected communities.
6. See that existing branding vanishes and record is left with no default community

Expected behavior

Set branding should set correctly the default community for the record and update the record landing page accordingly.