Include empty success test in JUnit output

[charliermarsh]

Summary

When Jenkins encounters a JUnit XML file that doesn't contain any test results, by default, it errors. You can configure Jenkins to treat empty files as "successful", but that opens the door to silently ignoring valid errors in your configuration or test running.

This PR modifies the JUnit reporter to include a single successful test if Ruff doesn't find any valid diagnostics.

Closes #4502.

Test Plan

Ran cargo run -p ruff_cli -- check foo.py --format junit --select F on an empty file:

<?xml version="1.0" encoding="UTF-8"?>
<testsuites name="ruff" tests="1" failures="0" errors="0">
    <testsuite name="ruff" tests="1" disabled="0" errors="0" failures="0" package="org.ruff">
        <testcase name="No errors found" classname="ruff">
        </testcase>
    </testsuite>
</testsuites>

I also uploaded this to a JUnit result visualizer online, and it looked reasonable:

[github-actions]

PR Check Results

Ecosystem

✅ ecosystem check detected no changes.

Benchmark

Linux

group                                      main                                   pr
-----                                      ----                                   --
linter/all-rules/large/dataset.py          1.00     14.2±0.07ms     2.9 MB/sec    1.00     14.2±0.10ms     2.9 MB/sec
linter/all-rules/numpy/ctypeslib.py        1.00      3.4±0.01ms     4.9 MB/sec    1.00      3.4±0.01ms     4.8 MB/sec
linter/all-rules/numpy/globals.py          1.00    421.6±1.75µs     7.0 MB/sec    1.01    424.5±1.27µs     7.0 MB/sec
linter/all-rules/pydantic/types.py         1.00      5.8±0.01ms     4.4 MB/sec    1.00      5.9±0.03ms     4.3 MB/sec
linter/default-rules/large/dataset.py      1.00      6.7±0.01ms     6.1 MB/sec    1.01      6.8±0.01ms     6.0 MB/sec
linter/default-rules/numpy/ctypeslib.py    1.00   1451.7±1.74µs    11.5 MB/sec    1.00   1453.2±1.49µs    11.5 MB/sec
linter/default-rules/numpy/globals.py      1.00    161.0±0.73µs    18.3 MB/sec    1.00    161.5±1.28µs    18.3 MB/sec
linter/default-rules/pydantic/types.py     1.00      3.0±0.01ms     8.5 MB/sec    1.01      3.1±0.01ms     8.4 MB/sec
parser/large/dataset.py                    1.00      5.4±0.00ms     7.6 MB/sec    1.21      6.5±0.01ms     6.3 MB/sec
parser/numpy/ctypeslib.py                  1.00   1062.6±0.76µs    15.7 MB/sec    1.16   1235.2±8.63µs    13.5 MB/sec
parser/numpy/globals.py                    1.00    109.1±0.23µs    27.0 MB/sec    1.12    122.8±0.13µs    24.0 MB/sec
parser/pydantic/types.py                   1.00      2.3±0.01ms    11.0 MB/sec    1.16      2.7±0.00ms     9.5 MB/sec

Windows

group                                      main                                   pr
-----                                      ----                                   --
linter/all-rules/large/dataset.py          1.00     17.4±0.43ms     2.3 MB/sec    1.00     17.4±0.39ms     2.3 MB/sec
linter/all-rules/numpy/ctypeslib.py        1.02      4.4±0.15ms     3.8 MB/sec    1.00      4.3±0.12ms     3.8 MB/sec
linter/all-rules/numpy/globals.py          1.00   504.0±18.78µs     5.9 MB/sec    1.02   514.7±23.72µs     5.7 MB/sec
linter/all-rules/pydantic/types.py         1.00      7.2±0.17ms     3.5 MB/sec    1.01      7.3±0.20ms     3.5 MB/sec
linter/default-rules/large/dataset.py      1.00      8.5±0.21ms     4.8 MB/sec    1.00      8.5±0.22ms     4.8 MB/sec
linter/default-rules/numpy/ctypeslib.py    1.00  1770.5±49.87µs     9.4 MB/sec    1.00  1770.7±36.91µs     9.4 MB/sec
linter/default-rules/numpy/globals.py      1.00    198.5±6.84µs    14.9 MB/sec    1.04   206.1±13.03µs    14.3 MB/sec
linter/default-rules/pydantic/types.py     1.00      3.8±0.09ms     6.7 MB/sec    1.00      3.8±0.08ms     6.7 MB/sec
parser/large/dataset.py                    1.01      6.9±0.11ms     5.9 MB/sec    1.00      6.8±0.10ms     6.0 MB/sec
parser/numpy/ctypeslib.py                  1.02  1322.6±55.20µs    12.6 MB/sec    1.00  1299.3±33.89µs    12.8 MB/sec
parser/numpy/globals.py                    1.01    135.1±5.09µs    21.8 MB/sec    1.00    133.3±4.24µs    22.1 MB/sec
parser/pydantic/types.py                   1.01      2.9±0.08ms     8.8 MB/sec    1.00      2.9±0.07ms     8.8 MB/sec