Hotfix for df size issue

[benartuso]

Temporary, will investigate further next week. This triggers a cache miss rather than a panic if the polars ops on the dataframe return nothing

Things I've seen so far @gschoeni

1. The df cache does still seem to write. I haven't yet verified the paths yet
2. Clearing out .oxen dir and pushing from new has the same issue, so it's definitely a recurring problem and not just a corrupt repo
3. I do think it's kinda weird that we store the df sizes for a commit under the version paths rather than the entry path 🤔