Hi @zejdlik ,
Currently, there is an known limits within Data Table control. If the column in your Data Table control has Blank value in your data source, you could not override the Blank value with other valid value, the Blank value would still remain Blank value:

As an alternative solution, I think the AddColumns function could achieve your needs. I have made a test on my side, please consider take a try with the following workaround:
Set the Items property of the Data Table as below:
RenameColumns(
DropColumns(
AddColumns(
'YourDataSource',
"Column1_Value",
If(IsBlank(Column_1), Column_2, Column1)
),
"Column_1"
),
"Column1_Value",
"Column_1"
)
then enable corresponding fields within your Data Table, the Column_1 would be populated with proper value.
Please consider take a try with above solution, then check if the issue is solved.
Best regards,