Empty values for different data types

Hi, I am trying to combine reports columns by adding null values.

For example:

Report 1: Name, Cost, Currency 

Report 2: Name, Time

Report 1+2: Name, Cost, Currency, Time

In order to use the combine function, I will need to add 1 empty column (Time) for report 1 and add 2 empty columns (Cost, Currency) for report 2.

Now my question is, what should I put in the field to keep the data type consistant?

'0000-00-00' for Time?

0.00 for Cost?

'' for Currency?

I tried these but it doesn't work. It would be great if you could let me know the empty values for different data types.