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.

Steps:

• 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