Let empty files be empty files (reprise)

[notpeter]

No longer add a "final newline" to empty files.
Empty files shouldn't need need trailing newlines.

Much simpler than previous version.

• Attempt to reland Let empty files be empty files #6839
• Which was reverted in subtree-push nightly-2026-06-10 #6939
• Fixes: rustfmt fails on an empty file; --check's diff doesn't result in properly formatted file #5364

[ytmimi]

@notpeter can you remind me why we're repurposing the */reorder_module tests for this PR and creating */reorder_module_2027? Is reordering the mods related to these whitespace changes? I'm a little confused by that right now.

Wouldn't it make more sense to create some new distinct test cases for what we're working on.

[notpeter]

When I was doing that PR, I was really bummed when it felt appropriate to duplicate the tree. In this PR, I've now switched from duplicated versions and instead added rustfmt-skip_children for the 2027 reorder tests. This way it will not attempt to reformat all the fixtures (which are empty files with newlines) and the rest of reorder_modules retains coverage under StyleEdition 2027. I think that leads to a simpler/clearer PR without having to duplicate a tree of dozens of empty files.

[ytmimi]

can we avoid making any changes to the reorder_modules test cases. It's still not clear to me why we're updating these existing test cases. I think we should create new distinct test cases that don't modify any of the old tests.

View changes since the review

[ytmimi]

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