Fix BiologicsReportTest - don't offer files with no content

[labkey-jeckels]

Rationale

The recent FileLike refactor broke the BiologicsReportTest by offering up R output files that didn't actually get generated. We've been creating 0 byte output files and then opt not to serve them to the client. This restores that behavior. As a followup, we might avoid creating these files in the first place.

Changes

• Restore check that sees if file exists and has content
• Misc code cleanup

Tasks 📍

• Manual Testing - N/A
• Needs Automation - verification run: https://teamcity.labkey.org/buildConfiguration/LabKey_Trunk_Premium_ProductSuites_Biologics_BiologicsPostgres/3762227

[labkey-alan]

This looks good to me, but as you noted, it does seem odd that we're even creating empty files.