[frontend] Make nullable first seen and last seen (#7268)

[Gwendoline-FAVRE-FELIX]

Proposed changes

• Remove require to first_seen and last_seen.
• Make the first value at null if the first value is undefined

Related issues

• https://github.com/OpenCTI-Platform/opencti/issues/7268

Checklist

• [ ] I consider the submitted work as finished
• [ ] I tested the code for its functionality
• [ ] I wrote test cases for the relevant uses case (coverage and e2e)
• [ ] I added/update the relevant documentation (either on github or on notion)
• [ ] Where necessary I refactored code to improve the overall quality

Further comments

[codecov[bot]]

Codecov Report

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

Project coverage is 67.44%. Comparing base (ffd3656) to head (26215c0). Report is 5 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #7483 +/- ## ========================================== + Coverage 67.36% 67.44% +0.07% ========================================== Files 563 563 Lines 69643 69680 +37 Branches 5889 5935 +46 ========================================== + Hits 46917 46993 +76 + Misses 22726 22687 -39 ```

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