fix: Accept empty result from sd-devices during export for backwards compatibility

[rocodes]

Description

Fixes #1594.

Test Plan

Check out this branch in your sd-app vm. Run the client and step through STR in linked issue. Ensure successful export, no errors in logs, and success confirmation message.

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

If these changes add or remove files other than client code, the AppArmor profile may need to be updated. Please check as applicable:

• I have updated the AppArmor profile
• No update to the AppArmor profile is required for these changes
• I don't know and would appreciate guidance

If these changes modify the database schema, you should include a database migration. Please check as applicable:

• I have written a migration and upgraded a test database based on main and confirmed that the migration is self-contained and applies cleanly
• I have written a migration but have not upgraded a test database based on main and would like the reviewer to do so
• I need help writing a database migration
• No database schema changes are needed

[rocodes]

(CI is failing due to an outdated dependency, not related to this PR)

[gonzalo-bulnes]

Looks good, still have to go through the test plan.

[gonzalo-bulnes]

• I can reproduce the error in main
• Exporting a file succeeds in this branch
• Exporting a file doesn't cause errors in sd-devices logs (~/.securedrop_export/logs/export.log)
• The success message is displayed in the dialog after the file has been exported