Below is a simple way to check if a string is null or empty in MS flow. It works by first testing if the string is null, and then checking if the string has 0 characters.
- Add a condition step
- Change the condition to OR
- Add the string variable, and first test to see if it equals null
- Secondly check if the formula
length(<varaible>)is equal to 0
If the condition is true, your variable is null or empty