fix csv parser when no separator detected

[SaInekK]

Description

convert_text_to_instruments previously called pd.read_csv(sep=None) for CSV files where no separator was detected. On pandas ≥ 2.3 this raises (the auto-detection branch was removed). This change guards the CSV-parse block with csv_sep is not None, so files with no detectable separator fall through to the txt branch instead of raising.

Fixes # (no upstream issue raised — happy to file one if you'd like a tracker)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Testing

• pytest tests/ → passes; restores the 6 test_remove_*_digits_from_csv tests that fail on main under pandas 2.3+.
• Manually tested with a one-column CSV (single header, no comma/semicolon/tab): no exception, parsed as text.

Test Configuration

• Library version: this branch
• OS: macOS 14
• Toolchain: Python 3.11, pandas 2.x

Checklist

• My PR is for one issue
• Self-reviewed
• No new warnings
• Tests pass locally