Fix optimize_dataframe_getitem bug

[rjzamora]

Closes #7692

In the current implementation of optimize_dataframe_getitem, it is possible for a collection name to be confused with a column selection. This PR includes a simple fix.

[jrbourbeau]

Thanks for fixing so quickly @rjzamora!

[jrbourbeau]

It wasn't immediately clear to me that this condition is what's needed for selecting out column projection layers. I wonder if there's something more direct we can check for here

[rjzamora]

I agree that it would be best to have a more-intuitive check here... Still thinking about this.

[rjzamora]

I guess this is more motivation for a formal getitem Layer... What we are looking for here is a Blockwise layer, based on operator.getitem, with the getitem-key (block.indices[1][0]) being a str or list. We do want the getitem key to be a literal (so block.indices[1][0] should be None), but the only time it will not be a literal is if the key is another layer/collection key. Therefore, we can either check that the getitem key is not a layer key, or we can check that the indice is None.

[jrbourbeau]

Thanks for the updates @rjzamora. I agree we might want to add some additional structure long-term to make this code easier to reason about. Though for now, the current set of changes look good and fix #7692

[rjzamora]

@jrbourbeau - Did you stil have concerns about this fix? I wasn't planning on making further changes here, but I certainly can if you have reservations or suggestions.