(#22772) Fix test for saving empty data

This test was failing due to a bug in the test, which didn't show up because
it was checking for too broad of an error.
(#22772) Allow a PUT with an empty body
(#22772) Add error checking to yaml format

This will raise a FormatError if asked to intern invalid YAML instead of an
unintelligible ruby error.
Merge pull request #1969 from pcarlisle/ticket/22772-empty-filebucket-save

Fix saving empty files to filebucket