test: add assertions to RelationService unit tests

[nova1128]

Describe your changes

Added assertions to the existing unit tests in RelationServiceTest.java
which previously had no assertions. Also mocked the save() return value
in createRelation test to properly verify the result.

Issue ticket number and link

Closes #4
#4

Checklist before requesting a review

• [x ] I have performed a self-review of my code
• [ x] If it is a core feature, I have added thorough tests.
• I have made corresponding changes to the documentation
• [ x] I used the given Project Style Guide

[nova1128]

this is my first contribution so maybe it will fall shaort on some paramters i would request uh to suggest me if i have done something wrong

[Mom0aut]

Looks good to me, thanks for your contribution 🚀