Default the warning checker's optional path prefix to an empty string #159338

The optional --path-prefix argument of Tools/build/check_warnings.py defaults to None. Parsing a warning then calls str.removeprefix(None), so the tool fails unless a prefix is supplied.

Reproducer, run with the interpreter built from the CPython checkout:

import subprocess
import sys
import tempfile
from pathlib import Path

with tempfile.TemporaryDirectory() as directory:
    output = Path(directory, 'compiler.txt')
    output.write_text(
        'source.c:1:2: warning: example [-Wunused]\n', encoding='utf-8')
    subprocess.run([
        sys.executable, 'Tools/build/check_warnings.py',
        '-c', str(output), '-t', 'gcc',
    ])

Expected: Omitting the optional prefix leaves file paths unchanged and the tool finishes normally.

Observed: A TypeError reports that the argument to removeprefix() must be a string, not None.

Environment: CPython main, Windows debug build.

Linked PRs

• gh-159338: Default the warning checker's optional path prefix to an empty string #159339