To identify is a string is empty use the following construct:
Pass a string value into a "Regex Extract" app that check for the regex
.+ (which will match any string that has at least one character).
Then, join the string array output from the regex app (this converts the string array to a string). Then run it through an "If" operator that checks if the string is not equal to
This works because the "Regex Extract" app will return
null if there was no match for the regex (in this case,
.+ which means the string is empty).