When using the Combine Columns data pipeline step to combine two columns, if one of the columns is null then the combined column will also be blank. This is because concatenation will ignore a null column and not show it in the result.
To address this, you can use a case statement formula in an Edit Column step prior to combining the columns.
An example case statement formula is as follows:
case when "Column2" is null then 0 else "Column2" end
This filled in the null values of Column2 with a 0, so the combined columns now show a result.