fix: Tweak rendering of `source` when it's a single ASCII whitespace

[BurntSushi]

This PR includes a small fix (that I am not at all sure about) and a regression test that fails on current master.

The input here is just a single ASCII whitespace with an annotation pointing to immediately after that whitespace. In current master, it gets rendered like this:

error: missing trailing newline at end of file
  |
1 | ...
  | ^ W292
  |
  = help: Add trailing newline

But I think the insertion of an ellipsis here is not quite right. I was certainly confused by the output.

I don't really understand the formatting code at all, but I believe the ellipsis is being inserted by this code:

if self.margin.was_cut_left() {
    // We have stripped some code/whitespace from the beginning, make it clear.
    buffer.puts(line_offset, code_offset, "...", *lineno_color);
}

And self.margin.was_cut_left() was returning true because computed_left was set to 18446744073709551593. This kind of value seems like a bug, although some margin values are explicitly initialized to usize::MAX, so maybe not.

Anywho, I tried removing the condition gating the setting of whitespace_margin, and that seems to fix this specific case without any other known regressions (including across all of ruff's test suite), but this was mostly a result of me feeling around in the dark here.