I just did some playing around with this and think I have found out why thats happening.
When we are parsing through a data source to find the options to display, we will read the rows and then the columns. There is a good explanation here (it says Google Sheets but is also applicable to Excel): https://support.prontoforms.com/hc/en-us/articles/4406631466260-Troubleshooting-Google-Sheets-Data-Source-Multiselect-Question-Showing-BlanksIt describes how we process the records in a data source and explains why would see a blank value in your drop down. In my test, I have 2 columns that are used for my options. If these 2 columns have different number of options, you will generate a blank value in the list of options because ProntoForms needs to read that row to get the options for the other column and just assumes that "blank" is an option.
Data Source with different number of options:The app:
Data source with the same number of options:Hope this helps!