BUG: `ItemCollection.to_dataframe()` excludes columns if they don't have any data in SharePoint

[widal001]

Overview

When querying a set of items from a SharePoint list if each of the items returned by a query are missing values for a column that column is excluded from the dataframe returned by ItemCollection.to_dataframe()

Steps to Reproduce

Steps to reproduce the behavior:

1. Instantiate a list class using SharePoint.get_list()
2. Use SiteList.get_items() to retrieve a set of items for which one column is blank for every item
3. Use ItemCollection.to_dataframe() to convert the collection returned to a dataframe
4. Print the list of columns in the dataframe

Expected Behavior

The resulting dataframe should include all fields passed to SiteList.get_items() even if there are no values set.

Screenshots

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Additional Context

The columns aren't returned becauseO365 excludes columns without a value from the Items.fields attribute. They need to be added either during the init step of ItemCollection or during the to_dataframe() method.