We shouldn't export empty folders when exporting

[LeviXIII]

Current Behaviour

When exporting with the wizard, in step 1 if you select a text only file and then a text + audio option, and then select an audio exporting option in step 2, you will be met with a warning saying that you will have an empty audio folder when exporting the text only file. When exporting after finishing step 3, you will have a folder that has a single NOTICE.txt file that just explains why it exists.

Acceptance Criteria

• Remove the warning about empty folders in the modal that pops up when selecting an audio exporting option in step 2 of the wizard.
• Change the exporting code to not export an empty audio file and thus, not give you the NOTICE.txt file.

Testing Checklist

Warning Removal (Step 2 modal)

• In Step 1, select a text-only file together with a text+audio file, advance to Step 2, and choose an audio export option → confirm the empty-folder warning modal no longer appears.
• Same setup but choose audio + timestamps → confirm no warning modal appears.
• Same setup but choose a non-audio export format → behavior unchanged (no spurious modals introduced).
• Select only text-only files (no audio cells anywhere) and pick an audio export option → confirm the wizard still handles this gracefully (either silently advances or shows the appropriate "no audio at all" guidance — not the removed empty-folder warning).
• Select only text+audio files and pick an audio export option → wizard advances normally, no modal.
• Verify the wizard's "Next Step" flow from Step 2 → Step 3 (or → Step 4 for non-Bible groups) works without the modal blocking advancement.

NOTICE.txt / Empty Audio Folder Removal

• Export a mix of text-only + text+audio files using an audio format → confirm the output directory contains no folder for the text-only file (or, if a folder exists, it is not empty and does not contain NOTICE.txt).
• Confirm no NOTICE.txt file is written anywhere in the export output.
• Export only text+audio files → audio folders for those files are populated normally (regression check).
• Export text+audio files where some cells have no recording → the file's audio folder still exists with recorded cells; per-cell "no audio recorded" reporting still works as before.
• Export a file that has audio attachments but where every cell is filtered out (e.g. by milestone selection excluding all milestones with audio) → confirm no empty folder + NOTICE.txt is left behind.
• Combined audio + non-audio bundle (when export format produces both) → text-only files still appear correctly in the non-audio output; only the audio side omits them.

Regression Testing

• Export progress reporter still completes cleanly (no stalled "preparing" / missing-file count mismatch caused by skipping the empty folder).
• Wizard step indicator and Back/Next buttons behave identically to before for all non-affected flows.
• Re-running an export to the same destination still overwrites/refreshes correctly and does not leave orphaned NOTICE.txt from a previous run (worth checking if you're testing on top of a folder that previously had one).