Fix warning when making a dataframe with mixed float/nan columns

[vanyae-cqc]

Dataframing quotas results in:

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.
pd.concat([item.df() for item in quota_list], ignore_index=True)