fix: 400/404 error on adding new users

[RadovanTomik]

Negotiator pull request:

Description:

Sometimes when a new user logged in for the first time they got 400/404 errors due to db conflicts. This PR should solve it.

Checklist:

Make sure you tick all the boxes bellow if they are true or do not apply:

• [ ] I have performed a self review of my code
• [ ] My code follows Google Java Code style
• [ ] I have made my code as simple as possible
• [ ] I have added unit tests and the code coverage has not decreased
• [ ] I have updated the documentation in all relevant places

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 22.05%. Comparing base (e276f5c) to head (7bd7789).

Files	Patch %	Lines
...on/security/auth/NegotiatorUserDetailsService.java	0.00%	3 Missing :warning:
...c/negotiator/api/controller/v3/UserController.java	0.00%	2 Missing :warning:
...guration/security/auth/CustomJWTAuthConverter.java	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #299 +/- ## ============================================= - Coverage 79.94% 22.05% -57.90% + Complexity 1006 316 -690 ============================================= Files 136 136 Lines 3017 3015 -2 Branches 163 163 ============================================= - Hits 2412 665 -1747 - Misses 505 2313 +1808 + Partials 100 37 -63 ``` | [Flag](https://app.codecov.io/gh/BBMRI-ERIC/negotiator/pull/299/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BBMRI-ERIC) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/BBMRI-ERIC/negotiator/pull/299/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BBMRI-ERIC) | `22.05% <0.00%> (-57.90%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BBMRI-ERIC#carryforward-flags-in-the-pull-request-comment) to find out more.

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