Add handling for blank metadata files

Currently you can crash the app if you upload a blank file.

We should make it so this fails more gracefully for users, and we should add test data and a UI test for this to make sure that it runs the initial data validation checks but fails.

Start by locating the upload handling and initial data validation entry points, then review the existing UI test structure. Add test data for a blank metadata file and a UI test that confirms validation runs before the upload fails gracefully. Done means the app no longer crashes and the test covers the expected failure.

Written by the indexing model from the issue text.