Fix double save in multipart requests

[ivarne]

8.5 preview versions included this bug where saving was done multiple times which caused there to be created multiple data elements with the same content.

This fixes the issue by moving the saving logic out of the loop of elements to save (which is correct behaviour for DataWriteProcessor), but somewhat changes behaviour in that IDataWriteProcessor gets called for all initialisations, also those that have empty change sets.

Review with hidden whitespace 😉

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)
• All tests run green

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[sonarcloud]

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 SonarQube Cloud