Performing find and replace in QGIS field calculator?

You could try a case when expression:

case when "Column_1" is NULL then  replace("Column_2",'Text2','Text1') else "Column_2" end

Using the 'or' expression, you can add multiple columns with NULL-values to the code.

case when "Column_1" is NULL or "Column_3" is NULL then  replace("Column_2",'Text2','Text1') else "Column_2" end

edit:

If you want to replace NULL, forget the replace-expression (it only works for strings and NULL is no string). Try this exact code:

case when "Feature" is NULL then '12_1' else "Feature" end

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