Fix: avoid mmap on empty ripgrep cache file and normalize input to rg

[bikemazzell]

#Fix: avoid mmap on empty ripgrep cache file and normalize input to rg (Issue #945)

Root cause:

• On POSIX systems, mmap() cannot map a zero-length file.
  When the ripgrep cache was empty, mmap raised “cannot mmap an empty file.”

Fix:

• In RipGrepCache.rebuild(), check cache file size; if 0, skip mmap and use b"" as the buffer.
• In ripgrep _fetch(), normalize cache buffer to a UTF‑8 string before passing to subprocess.check_output to handle mmap bytes, empty buffers, and Windows string fallback consistently.

Tests:

• Added tests/test_issue_945.py:
  • test_mmap_empty_file_issue_945: empty cache no longer raises and returns empty results.
  • test_non_empty_mapping_unchanged: behavior for non-empty files remains unchanged.

[timalander]

Any movement on this PR? Also running into this error on Mac