How to Fill blank Fields with other column QGIS

Welcome to the wonders of COALESCE. Try this:

COALESCE( "pole_type_" , "pole_type" )

It will return the first non-null value in a list. If they are all null, it will return null.

You can use:

CASE WHEN "pole_type" IS NULL THEN "pole_type_" ELSE "pole_type" END