FURB105 fix should be unsafe when it removes a `sep` with a possibly incorrect type

[dscorbett]

Summary

print’s sep argument must be None or a str, even if it is not used, or it raises an error. The fix for print-empty-string (FURB105) should be marked unsafe when it removes a sep whose type it can’t prove is correct, because that can change the program’s behavior. Example:

$ cat >furb105.py <<'# EOF'
def p(sep):
    print(1, sep=sep)
p(0)
# EOF

$ python furb105.py 2>&1 | tail -n 1
TypeError: sep must be None or a string, not int

$ ruff --isolated check furb105.py --select FURB105 --fix
Found 1 error (1 fixed, 0 remaining).

$ python furb105.py
1

Version

ruff 0.14.0 (beea8cd 2025-10-07)