Canvas ingestion bug fix

[shanbady]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/9269

Description (What does it do?)

This PR resoolves an issue where if a canvas course does not have any modules. The ingestion process silently fails even if there are tutor problems and/or files to process as contentfiles.

How can this be tested?

1. checkout main
2. download the example canvas archive which is missing modules_meta.xml
3. place the archive relative to your learn repo
4. open a django shell and attempt to process it with the following script and see it fail:

from learning_resources.etl.canvas import run_for_canvas_archive,transform_canvas_content_files
from pathlib import Path
zipfile = Path("broken-canvas.zip")
_, run = run_for_canvas_archive(zipfile, "tmp", True)
list(transform_canvas_content_files(zipfile, run,{}, overwrite=True))

5. checkout this branch and do the same and note that it succeeds

[abeglova]

👍