FIO-7589: hotfix for no return crash

[brendanbond]

Link to Jira Ticket (if applicable)

https://formio.atlassian.net/browse/FIO-7589

Description

If no files were uploaded, the server will return a 400 but continue execution. Someone forgot a return statement if no files were uploaded.

Dependencies

n/a

How has this PR been tested?

n/a

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• My changes include tests that prove my fix is effective (or that my feature works as intended)
• New and existing unit/integration tests pass locally with my changes
• Any dependent changes have corresponding PRs that are listed above

[AlexeyNikipelau]

LGTM