[keyserver] Make sanitizeInput work even if validation fails

Summary:
We actually only use sanitizeInput when validation fails, so it's important that it works in this case.

See context here.

Test Plan: I added a test case to validation-utils.test.js that was failing before the changes in this diff

Reviewers: kamil, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12391