Surprisingly there is no standard function is SharePoint Designer to set a string field to empty. After many workarounds I realized that the easiest way to do this is through the ‘Extract Substring of String from Index with Length’ action, specifying the length as 0.
For example, assuming that we wanted to set the string ‘String01’ to empty, we could use the following ‘Extract string …’ statement:
Copy from Variable:String01, starting at 0 for 0 characters (Output to Variable:String01)
Since 0 characters are being copied, the resulting string ‘String01’ is empty
