Raise clear error when no flight images present for ILS correction

[BeesualDrone]

Add error handling for empty image DataFrame in ILS correction.

What?

Raises an explicit FileNotFoundError when image_df is empty after the calibration/non-calibration split in io.create_cal_df, before reaching the ILS merge step.

Why?

If ils_present=True and the Input Path contains only calibration panel images (no flight images), image_df ends up empty after the split. The subsequent merge with band_avg_ils produces 0 rows, and DataFrame.apply(axis=1) on that empty, multi-column frame returns the original DataFrame instead of an empty Series — causing band_df["ils_scaling_factor"] = ... to fail with a confusing internal pandas error instead of a clear, actionable message.

Full reproduction and root-cause writeup: Closes #77

PR Checklist

• Merged latest master
• Updated version number
• All private git packages are at their newest version in both pyproject.toml and environment.yml
• All git package version numbers match between pyproject.toml and environment.yml
• dist/ImageryCorrector.exe and dist/GetCorrectionsCsv.exe have been rebuilt

Breaking Changes

None. This only adds a new, earlier validation error for an input configuration that previously failed anyway, just with a less clear message.