Stop concatenating null dataframes

[zaneselvans]

In the main PUDL ETL we are currently getting tens of thousands of warnings like:

/Users/zane/miniforge3/envs/pudl-dev/lib/python3.11/site-packages/ferc_xbrl_extractor/xbrl.py:169: FutureWarning: The behavior of DataFrame concatenation with empty or all-NA entries is deprecated. In a future version, this will no longer exclude empty or all-NA columns when determining the result dtypes. To retain the old behavior, exclude the relevant entries before the concat operation.

We should probably stop attempting to do these null concatenations, lest it break down in the future. Or maybe we can just assign explicit data types? Or suppress the warnings so we only get it 1 time, instead of 17,000 times?

For the moment these warnings are being ignored completely. See #170