tidy: Add a check for empty `.stderr` and `.stdout` files in UI test directories

[petrochenkov]

Tidy check in https://github.com/rust-lang/rust/blob/master/src/tools/tidy/src/ui_tests.rs makes sure that we don't have stray testname.stderr/testname.stdout files without a corresponding testname.rs.

It would be useful to add one more check into the same file making sure that testname.stderr/testname.stdout files are not empty.
Such empty files can still pass testing for compile-pass tests with no output so they can get into the repo accidentally, but they are not necessary and can be removed.

[yaahc]

I'd like to take this issue.