When importing data using OG Pro by going to Data>Import data you will see a tick option for "Overwrite with empty values". You only need to use this if the file you're importing contains all fields even of they contain no data. If you then tick the option the fields with no data will overwrite the same fields in the project with blanks, even if they had data.
Generally in most cases you wont need to use this option unless you need to change original values for blanks with the import.