fix: handle empty files in --only-allowlisted scan

[ReGenDevOps]

Problem

detect-secrets scan --only-allowlisted crashes with a ValueError when it encounters an empty file:

ValueError: not enough values to unpack (expected 2, got 0)

Traceback:

File "detect_secrets/core/scan.py", line 229, in _scan_for_allowlisted_secrets_in_lines
    line_numbers, lines = zip(*lines)
ValueError: not enough values to unpack (expected 2, got 0)

Root Cause

_scan_for_allowlisted_secrets_in_lines() calls zip(*lines) directly. When lines is an empty iterable (from an empty file or an empty diff chunk), zip() returns an empty tuple which cannot be unpacked into two variables.

Fix

Materialize the iterable into a list first and return early if empty. This is semantically correct — an empty file cannot contain any allowlisted secrets.

lines_list = list(lines)
if not lines_list:
    return

line_numbers, lines = zip(*lines_list)

Testing

• Added test_handles_empty_file_for_allowlisted_scan to tests/core/scan_test.py
• Verified on Python 3.14.2 with detect-secrets 1.5.0
• Tested with both empty .py files and full project scans

Environment

• Python 3.14.2
• detect-secrets 1.5.0
• macOS 15 (Apple Silicon)