SUMIFS Data Validation Blank Select All

Hi,
I am trying to work this out and haven't had any luck. I have a Resource Planning Template that I found on the internet and I am trying to understand what is being done...

If you have a look on the 'Resource Summary' page, in cell C3 there is a data list where when the project is selected it filters the numbers below. I understand that it is using SUMIFS to achieve this. I have tried to recreate this sheet in a seperate workbook, but everytime I use the SUMIFS, when C3 is empty, nothing pops up, whilst here it has gone ahead and has selected all the projects.

For e.g. under Person 1 - even though C3 is empty, it is still showing the aggregate for all the hours. I have tried everything including copying the formula but unfortunately I have not had much luck.

Would you be able to explain how this has been achieved? I essentially want to recreate the spreadsheet where having C3 as blank will still show all the numbers and it will only then filter thereafter.

The document has been attached for reference. Thank you