str_replace_tool returns OK for empty files without checking old_str

Description

DeerFlow's `str_replace_tool` at `tools.py:879` has an early return: `if not content: return 'OK'`. This bypasses the 'old_str not found' check for empty files, returning success even when the intended replacement string was not present.

Reproduction

# tools.py:879:
if not content:
    return 'OK'  # Bypasses the not-found check entirely