Check for empty datetime strings

[jacobdadams]

~arcgis can't convert a pd.NA value to a datetime value when converting a sdf to feature set. Ideas:~

~- Add another check ala nullable int shapely checker~ ~- Convert these to empty strings via fillna? Would empty strings convert to the proper feature set data?~ ~- Don't allow the nullable string type when checking types for esriFieldTypeDate- forcing user to not use this type~

~Which would be easiest for user?~

AGOL can't parse an empty string '' to a date field.

• It can handle null values in the pandas nullable string dtype
• it can handle None in an object dtype
• it can handle NaT in a datetime[ns] dtype

[jacobdadams]

Maybe we require all date fields to be converted to a datetime[ns] before appending?