swagger: note that the inviting user is nullable in invitationsView

[zenovich]

fixes #1133

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (ca7dec8) to head (5cd3b5a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1136 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 239 239 Lines 14542 14542 ========================================= Hits 14542 14542 ```

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[zenovich]

[smadbe]

I assumed you have tested it

Ah, I had not seen your screenshot :+1: