For the textSummary property, I would have thought checking for an empty string would work. Note that when you see a value of "value": "" that doesn’t mean that you would enter double quotes in the condition (""). Instead, it would mean a completely empty field (you’ll likely need to enter something in the field and then delete it so it’s an empty string):
Note that this is not the same as checking for the existence of a field. In this case, the textSummary field should always be included in the response… it’s just that the field will be an empty string when there is no match.
If you wanted to check for the existence of a field, you could use an expression with the isEmpty() function to set a boolean. This post and this post are good examples showing the isEmpty() function in action.