[frontend] Fix Empty TAXII Ingester User Reverts to Previous User instead of System (#4813)

[helene-nguyen]

Proposed changes

• Handle empty value

Related issues

• Empty TAXII Ingester User Reverts to Previous User instead of System #4813

Checklist

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

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[SouadHadjiat]

When I empty the input, I can now see an empty user in the list, and if I just empty the input and close the update panel, when I reopen it it still displays my old user.

The only way I can set back to System user, is to select in the list the empty user.

Also, When I create a new ingester, and try to set a user, I will always see this empty user in the list first