Time to do this: 1 minute
I’ve probably Google’d this dozens of times since I started using Microsoft Flow with SharePoint Online. The top hit on Google isn’t very helpful, but the solution is actually pretty simple. So instead of doing this over and over again, I’m just posting what I’ve found works - at least for most column types.
If you want to have a workflow check to see if a column is empty and then do something, you can just use the expression builder. There’s a predefined value in there for “null.” Throw that puppy on the right side of your condition and test away.
Once you’ve got that easy step in place, go ahead and test your flow on empty columns.
Note - I have noticed that this doesn’t always work when using it on a multi-step condition, especially when the value you’re trying to check for is on a multi-select column.