Aircraft Importer, Empty FIN field causes problems

[FatihKoz]

While importing aircraft, empty fin field causes sql integrity constraint violation error.

Version
7.0.0-dev+231022.d69a3b

To Reproduce
Steps to reproduce the behavior:

1. Export some of your aircraft (not subfleet)
2. Check the fin fields in the csv, they should be empty
3. Import the file back
4. See error

Expected behavior
Empty fields should not cause unique checks to fail and model should not return empty strings, they should return null

Additional context
Example aircraft.csv

subfleet,iata,icao,hub_id,airport_id,name,registration,fin,hex_code,mtow,zfw,status
A20N-AJET,20N,A20N,,LTFJ,TC-LUK,TC-LUK,,,79000,,A
A20N-AJET,20N,A20N,,LTFJ,TC-LUL,TC-LUL,,,79000,,A
A20N-AJET,20N,A20N,,LTCG,TC-LUN,TC-LUN,,,79000,,A
A20N-AJET,20N,A20N,LTAC,LTBJ,TC-LUO,TC-LUO,,,79000,,A
A20N-AJET,20N,A20N,LTAC,LTAC,TC-LUV,TC-LUV,,,79000,,A
A20N-AJET,20N,A20N,LTAC,LTAC,TC-LUY,TC-LUY,,,79000,,A
A20N-AJET,20N,A20N,LTAC,LTAC,TC-LUZ,TC-LUZ,,,79000,,A

Import result

Error in row 3: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'aircraft_fin_unique' (Connection: mysql, SQL: update `aircraft` set `hub_id` = , `fin` = , `hex_code` = 4df98ce6, `zfw` = , `aircraft`.`updated_at` = 2023-10-23 17:38:55 where `id` = 827)
Error in row 4: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'aircraft_fin_unique' (Connection: mysql, SQL: update `aircraft` set `hub_id` = , `fin` = , `hex_code` = 4cb04e1e, `zfw` = , `aircraft`.`updated_at` = 2023-10-23 17:38:55 where `id` = 828)