Fix responce when entering a non-existent user

[KizerovDmitriy]

GreenCity PR

Summary Of Changes 🔥

• add check, when name is empty string.
• rewrote test logic.

Issue Link 📋

#6398

PR Checklist Forms

(to be filled out by PR submitter)

• Code is up-to-date with the dev branch.
• You've successfully built and run the tests locally.
• There are new or updated unit tests validating the change.
• JIRA/ Github Issue number & title in PR title (ISSUE-XXXX: Ticket title)
• This template filled (above this section).
• Sonar's report does not contain bugs, vulnerabilities, security issues, code smells or duplication
• NEED_REVIEW and READY_FOR_REVIEW labels are added.
• All files reviewed before sending to reviewers

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud