An example file would be useful...
As a note - Tableau will use the first data item in the column to determine the fields type
Try adding the attached Excel to Tableau, this is how mine looks
Hope you can see, Its correctly picking up the data type for the columns.
I would ensure the first row of your data is good, before you link to Tableau, or put it in a staging Excel sheet first, sort it out, then connect.
With regard to the nulls, ZN() function.
The more information you can provide, the better guidance we can give!
Book1.xlsx 8.0 KB