Fix dataframe concatenation in pre-processing step

During the pre-processing step we get a warning because of empty or all-NA entries.

dsa-tdb/dsa_tdb/fetch.py:408: 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.
  df_out = pd.concat((df_out, records_out), ignore_index=True, sort=True, verify_integrity=True)